Heterogeneous, composite, carbonaceous catalyst system and methods that use catalytically active gold

ABSTRACT

Heterogeneous catalyst systems, methods of making these systems, and methods of using these systems, wherein catalytically active gold is deposited onto composite support media. The composite support media is formed by providing nanoporous material on at least a portion of the surfaces of carbonaceous host material. In representative embodiments, relatively fine, nanoporous guest particles are coated or otherwise provided on surfaces of relatively coarser activated carbon particles. Catalytically active gold may be deposited onto one or both of the guest or host materials either before or after the guest and host materials are combined to from the composite host material. PVD is the preferred catalyst system of depositing gold.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/275,416, filed Dec. 30, 2005, which claimed the benefit of U.S.Provisional Patent Application Ser. No. 60/641,357, filed Jan. 4, 2005,the disclosures of both of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to gold-based catalyst systems in whichcatalytically active gold is provided on composite support media.

BACKGROUND OF THE INVENTION

Carbon monoxide is a toxic gas formed by incomplete burning of organicmaterials. Carbon monoxide combines with blood hemoglobin to formcarboxyhemoglobin which is ineffective at transporting oxygen to bodycells. Inhalation of air containing 1-2% (10,000 to 20,000 ppm) CO byvolume will cause death within several minutes. CO concentrations higherthan 1200 ppm are considered immediately dangerous to life and health bythe U.S. National Institute of Occupational Safety and Health (NIOSH).

CO is responsible for many of the fatalities in fires. It is alsoencountered in mining operations in which explosives are used inconfined spaces. CO is also present in the exhausts of gasoline ordiesel powered internal combustion engines. Poorly operating engines,machinery, heating equipment, ventilation equipment, air conditioningequipment, and other equipment may also output CO, contaminating the airin buildings and vehicles. Consequently, there is a strong need forprotection against CO in these and other environments in which personscould encounter the gas.

Firefighters and other emergency response personnel have been equippedwith self-contained respirators using compressed air or oxygen incylinders to provide protection against CO. These devices tend to beheavy, bulky, expensive and require special training for effective use.It is not feasible to equip everyone in an area with such devices.

A fire or other sudden unexpected release of carbon monoxide in abuilding, public place, vehicle, or the like may require thatindividuals quickly escape from an area containing dangerousconcentrations of the gas. In these situations, an easy-to-use,lightweight respirator or mask equipped with media capable of protectingagainst carbon monoxide would be desirable.

Protection against CO is also desirable in the cabin environment of acar, truck, rail-borne vehicle, marine vessel, or other mode oftransport. In many heavily congested traffic areas and in tunnels,elevated levels of CO can develop from the accumulation of exhaustemissions. Typically, the CO levels encountered are usually less than200 to 300 ppm, but even these CO levels can cause headaches, dizzinessand nausea to drivers and passengers. In these applications, largevolumes of gas and high flow rates can be encountered. Thus, theresidence time of the cabin air on the catalyst is short, being lessthan 0.05 seconds and even less than 0.03 seconds. It is thereforedesirable to have a catalyst that can also remove CO under theseconditions.

However, the low boiling point and high critical temperature of CO makeits removal by physical adsorption very difficult when the CO is presentat room temperature. Conventional gas mask canisters and filters basedon activated carbon adsorbents have been relatively useless as apractical matter against high concentrations of carbon monoxide.

Catalytic oxidation to carbon dioxide is one feasible method forremoving carbon monoxide from air at the high concentrations and flowrates required for individual respiratory protection. However, most COoxidation catalysts are only active at temperatures of 150° C. orhigher. This is true even though oxidation to CO₂ is thermodynamicallyfavored. Very few CO oxidation catalysts are active at room temperatureor below. A catalyst useful for respiratory protection against COdesirably functions at low temperatures.

Two types of catalysts that are known for low temperature CO oxidationinclude transition metal oxides (mostly mixed oxides of Cu, Mn, and/orCo) and supported noble metal catalysts. One widely used transitionmetal oxide for low temperature CO oxidation is hopcalite. Hopcalite isa mixed oxide of manganese and copper developed during World War I bythe U.S. Bureau of Mines and the Chemical Warfare Service of the U.S.Army [Lamb, Bray, and Frazer, J. Ind. Eng. Chem., 12, 213 (1920)].Hopcalite is a very active catalyst for CO oxidation even attemperatures as low as −20° C. The major disadvantage of hopcalite isthat its capability for CO oxidation is quickly destroyed by water vaporin the air. This means that a respirator filter with a hopcalitecatalyst must include a drier bed on the inlet side of the filter. Theuseful life of the respirator filter is determined by the capacity andefficiency of the drier bed. Even a filter designed for short term use(˜30 minutes) at high breathing rates will require a desiccant bed oflarger volume than the catalyst bed itself. Hopcalite is commerciallyavailable from Cams Chemical Company, 315 Fifth Street, Peru, Ill. 61354USA under the designation Carulite 300.

Catalytic oxidation of CO over supported platinum group metals (mostoften Pt, Pd, Rh, Ru, and Ir) has been known for many years. However,most of these catalysts are only active at temperatures around 150° C.

In recent years, supported platinum group metal catalysts have beendeveloped that function at lower temperatures. In addition to a platinumgroup metal, these catalysts may also contain so-called “reducible metaloxides” such as SnO_(x), CeO_(x), and FeO_(x). It is thought that thereducible oxides provide sites that dissociatively adsorb O₂, therebypromoting low temperature CO oxidation. U.S. Pat. No. 4,536,375 andPublished UK Patent Application GB 2,141,349 discuss these catalysts andtheir use in respiratory protection devices. A low temperature COoxidation catalyst of this type is commercially available from MolecularProducts Ltd, Mill End, Thaxted, Essex CM6 2LT, United Kingdom under thedesignation Sofnocat® 423. It contains platinum, palladium, and SnO₂.

These platinum-based catalysts are much more tolerant of water vaporthan is hopcalite. However, operation at high relative humidity (RH)with low CO inlet concentrations results in capillary condensation ofwater vapor in the micropores of the catalyst support (usually aluminaor silicagel). This causes slow loss of activity as access to activesites is blocked by condensed water. A significant disadvantage of thesecatalysts is the high loading of expensive platinum group metalnecessary to meet the requirements for respiratory protection againstCO.

It has been observed that nanoislands of very finely divided gold onreducible oxide supports are very active for CO oxidation at lowtemperature. At ambient to sub-ambient temperatures, the best goldcatalysts are considerably more active for CO oxidation than the mostactive promoted platinum group metal catalyst known. Gold is alsoconsiderably cheaper than platinum. Catalytically active gold, though,is quite different from the platinum group metal catalysts discussedabove. The standard techniques used in the preparation of supportedplatinum group metal catalysts give inactive CO oxidation catalysts whenapplied to gold. Different techniques, therefore, have been developedfor deposition of finely divided gold on various supports. Even so,highly active gold catalysts have been difficult to preparereproducibly. Scaleup from small lab preparations to larger batches hasalso proved difficult.

These technical challenges have greatly hindered the industrialapplication of gold catalysts. This is unfortunate since the very highactivities of gold catalysts for CO oxidation at ambient and sub-ambienttemperatures and their tolerance for high water vapor concentrationsmake them otherwise strong candidates for use in respiratory protectionfilters and in other applications in which oxidation of CO would bedesired.

Because ultra-fine particles of gold generally are very mobile andpossess large surface energies, ultra-fine particles of gold tend tocoagulate easily. This tendency to coagulate makes ultrafine gold hardto handle. Coagulation also is undesirable inasmuch as the catalyticactivity of gold tends to fall off as its particle size increases. Thisproblem is relatively unique to gold and is much less of an issue withother noble metals such as platinum (Pt) and palladium (Pd). Thus, it isdesired to develop methods to deposit and immobilize ultra-fine goldparticles on a carrier in a uniformly dispersed state.

Known methods to deposit catalytically active gold on various supportsrecently have been summarized by Bond and Thompson (G. C. Bond and DavidT. Thompson, Gold Bulletin, 2000, 33(2) 41) as including (i)coprecipitation, in which the support and gold precursors are broughtout of solution, perhaps as hydroxides, by adding a base such as sodiumcarbonate; (ii) deposition-precipitation, in which the gold precursor isprecipitated onto a suspension of the pre-formed support by raising thepH, and (iii) Iwasawa's method in which a gold-phosphine complex (e.g.,[Au(PPh₃)]NO₃) is made to react with a freshly precipitated supportprecursor. Other procedures such as the use of colloids, grafting andvapor deposition, have met with varying degrees of success.

These methods, however, suffer from difficulties aptly described by Wolfand Schuth, Applied Catalysis A: General, 2002, 226 (1-2) 1-13(hereinafter the Wolf et al. article). The Wolf et al. article statesthat “[a]lthough rarely expressed in publications, it also is well knownthat the reproducibility of highly active gold catalysts is typicallyvery low.” The reasons cited for this reproducibility problem with thesemethods include the difficulty in controlling gold particle size, thepoisoning of the catalyst by ions such as Cl, the inability of thesemethods to control nano-sized gold particle deposition, the loss ofactive gold in the pores of the substrate, the necessity in some casesof thermal treatments to activate the catalysts, inactivation of certaincatalytic sites by thermal treatment, the lack of control of goldoxidation state, and the inhomogeneous nature of the hydrolysis of goldsolutions by the addition of a base.

In short, gold offers great potential as a catalyst, but thedifficulties involved with handling catalytically active gold haveseverely restricted the development of commercially feasible,gold-based, catalytic systems.

German Patent Publication DE 10030637 A1 describes using PVD techniquesto deposit gold onto support media. The support media described in thisdocument, though, are merely ceramic titanates made under conditions inwhich the media would lack nanoporosity. Thus, this document fails toindicate the importance of using nanoporous media to supportcatalytically active gold deposited using PVD techniques. InternationalPCT Patent Publications WO 99/47726 and WO 97/43042 provide lists ofsupport media, catalytically active metals, and/or methods for providingthe catalytically active metals onto the support media. These twodocuments, however, also fail to appreciate the benefits of usingnanoporous media as a support for catalytically active gold depositedvia PVD. Indeed, WO 99/47726 lists many preferred supports that lacknanoporosity.

Relatively recently, very effective, heterogeneous catalyst systems andrelated methodologies using catalytically active gold have beendescribed in assignee's co-pending United States patent applicationhaving U.S. Ser. No. 10/948,012, titled CATALYSTS, ACTIVATING AGENTS,SUPPORT MEDIA, AND RELATED METHODOLOGIES USEFUL FOR MAKING CATALYSTSYSTEMS ESPECIALLY WHEN THE CATALYST IS DEPOSITED ONTO THE SUPPORT MEDIAUSING PHYSICAL VAPOR DEPOSITION in the names of Larry Brey et al., andfiled Sep. 23, 2004, the entirety of which is incorporated herein byreference (hereinafter referred to as Assignee's Co-pendingApplication). In particular, Assignee's Co-pending Application describesproviding catalytically active gold on a composite support derived fromrelatively fine titania particles (referred to as guest material) thatat least partially coat the surfaces of relatively large aluminaparticles (referred to as host material). These composite systemsprovide excellent catalytic performance with respect to CO oxidation.However, improvements are still desired. Notably, it would be desirableto provide gold-based catalyst systems that demonstrate a fasterresponse to changes in incident CO challenges. It is further desirablefor gold-based catalysts to provide longer lasting protection againstCO. It is also desirable to use such catalysts in respiratory protectionsystems that provide protection against not only CO but other airbornecontaminants as well.

SUMMARY OF THE INVENTION

The present invention relates to heterogeneous catalyst systems, methodsof making these systems, and methods of using these systems, whereincatalytically active gold is deposited onto composite support media. Thecomposite support media is formed by providing nanoporous material on atleast a portion of the surfaces of carbonaceous host material. Inrepresentative embodiments, relatively fine, nanoporous guest particlesare coated or otherwise provided on surfaces of relatively coarseractivated carbon particles. Catalytically active gold may be depositedonto one or both of the guest or host materials either before or afterthe guest and host materials are combined to from the composite hostmaterial.

Carbonaceous material, especially activated carbon particles, is apreferred host material for a variety of reasons. Firstly, compositecatalysts incorporating carbonaceous host material have shown asignificantly faster response time for CO oxidation when challenged witha 4×, stepwise increase in CO as compared to composite catalystsincorporating alumina host material. Composite catalysts incorporatingcarbonaceous host material also provide very long-lasting protectionagainst CO. The composite catalyst system of the present invention alsocatalytically oxidizes the CO in ambient air streams, although thecatalytic oxidation of CO will tend to generate heat and raise thetemperature of the composite during the course of catalytic oxidation.

Unlike some other host material, carbonaceous host material also canfunction as a filtering medium for organic gases and vapors, therebyfiltering organic contaminants from an air or other gaseous stream.Carbonaceous material can also be impregnated with one or moreimpregnants (described further below) to provide additional filteringcapabilities. According to conventional wisdom, one generally mightexpect adding guest material to be accomplished at the expense of one ormore of such other filtering abilities at least to some significantdegree. This is based partly upon the circumstances that a carbonaceousmedium can only incorporate a finite quantity of one or more reactiveimpregnants before its capacity to hold additional impregnants isessentially saturated. Thus, one typically must balance and compromiseamong several desired objectives when deciding what kinds and how muchof different impregnants are to be incorporated into a support. Forinstance, if one desires to add more triethylenediamine (TEDA)impregnant to a carbonaceous support to provide additional protectionagainst cyanogen chloride, the extra TEDA present could reduce to somedegree the amount of organic protection that might otherwise be providedif a lesser amount of TEDA were to be used. Surprisingly, however,providing guest material on the carbonaceous host has very little, ifany, practical impact upon the organic filtering ability of thecarbonaceous host material with respect to inherent filteringcapabilities (e.g., the ability to protect against organic vapors) orwith respect to filtering capabilities provided by one or moreimpregnants incorporated into or on the carbonaceous material. In short,guest material may be added to a carbonaceous host without undulylimiting other filtering benefits offered by the host.

As another advantage of carbonaceous host material, these materialsreadily associate with guest materials such as titania particles.Carbonaceous material also has a much lower density than some otherhosts such as alumina. Filters containing composites with carbonaceoushosts thus weigh less than an equal volume of composites with aluminahosts. Carbonaceous host material is also inexpensive, makingcarbonaceous host material very economical to use.

The gold-based catalyst systems of the present invention have excellentcatalytic performance. These systems would find application in the areaof CO abatement in the form of personal, vehicle and buildingprotection, catalysts and catalyst supports for the purification ofexhaust gases from internal combustion engines, removal of CO from fuelcell feedstocks, and in catalyzing other oxidation reactions such as theoxidation of carbonaceous soot in diesel exhaust streams and theselective oxidation of organic compounds. For instance, the gold-basedcatalyst systems would be suitable as catalyst systems for the catalyticoxidation of unsaturated and saturated hydrocarbons. The termhydrocarbon means unsaturated or saturated hydrocarbons such as olefinsor alkanes. The hydrocarbon can also contain heteroatoms like N, O, P, Sor halogens. The organic compounds to be oxidized may be acyclic,monocyclic, bicyclic, or polycyclic and may be mono-olefinic,di-olefinic, or poly-olefinic. The double bonds in compounds with two ormore double bonds may be conjugated or non-conjugated.

In one aspect, the present invention relates to a method of making aheterogeneous catalyst system. Catalytically active gold is incorporatedinto a composite, nanoporous support medium derived from ingredientscomprising guest material and carbonaceous host material. In preferredembodiments, the catalytically active gold is deposited onto the supportmedium under conditions such that the system comprises 0.005 to 5 weightpercent gold based on the total weight of the gold and the supportmedium. It is also preferred that the host material comprises activatedcarbon particles and that the guest material comprises titania. Themethod may further comprise the step of impregnating a water solublesalt onto the host material, wherein said impregnation occurs prior tothe gold deposition.

In another aspect, the present invention relates to a heterogeneouscatalyst system. The system includes a nanoporous, composite supportmedium derived from ingredients comprising relatively fine guestparticles and relatively coarse carbonaceous particles. Catalyticallyactive gold is deposited onto the support medium, preferably usingphysical vapor deposition.

In another aspect, the present invention relates to a method ofoxidizing CO. A heterogeneous catalyst system is provided. The systemincludes a nanoporous, composite support medium derived from ingredientscomprising relatively fine guest particles and relatively coarsecarbonaceous particles; a promoting amount of an alkali metal saltpresent on the composite support medium; and catalytically active goldpresent on the composite support medium. The heterogeneous catalystsystem is caused to catalytically contact the CO.

In another aspect, the present invention relates to a method of making acatalyst system. A plurality of relatively fine, nanoporous guestparticles is incorporated onto relatively larger, carbonaceous hostparticles to form a plurality of composite particles. Catalyticallyactive gold is deposited onto the composite particles using physicalvapor deposition.

In another aspect, the present invention relates to a method of making acatalyst system. Catalytically active gold is deposited onto a pluralityof relatively small, nanoporous guest particles using physical vapordeposition. After the gold deposition, the nanoporous guest particlesare incorporated onto a plurality of relatively large, carbonaceous hostparticles.

In another aspect, the present invention relates to a method of making aheterogeneous catalyst system. A plurality of relatively fine,nanoporous particles and a plurality of relatively coarser, carbonaceousparticles are incorporated into a plurality of composite particles.Catalytically active gold is deposited onto the composite particlesusing physical vapor deposition.

In another aspect, the present invention relates to a heterogeneous,catalyst system, comprising a plurality of composite, catalyticallyactive particles, wherein said composite, catalytically active particlesare derived from ingredients comprising relatively fine particles and,relatively coarser, carbonaceous particles, and wherein the compositeparticles comprise catalytically active gold deposited onto therelatively fine particles using physical vapor deposition.

In another aspect, the present invention relates to a heterogeneous,catalyst system, comprising a plurality of relatively fine particlesdeposited onto a relatively coarser, carbonaceous support medium, andwherein the system comprises catalytically active gold.

In another aspect, the present invention also relates to respiratoryprotection system comprising a heterogeneous catalyst system made inaccordance with the principles of the present invention and/orincorporating features of a heterogeneous catalyst system of the presentinvention. The protection system may constitute all or a portion of apersonal respiratory protection system, a building respiratoryprotection system, a vehicle respiratory protection system, a mask, anescape hood, an air purification device, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a TEM image of a cross-section of a representative catalystsurface of the present invention (material of Example 3 of Assignee'sCo-Pending Application cited above).

FIG. 2 is a schematic perspective view of the apparatus of FIG. 2.

FIG. 3 is a schematic side view of an apparatus for carrying out a PVDprocess for depositing catalytically active gold onto a support.

FIG. 4 a schematically shows a testing system that was used to subjectsamples to CO challenges in order to assess catalytic characteristicsfor oxidizing CO.

FIG. 4 b schematically shows a testing system that was used to subjectsamples to CO challenges in order to assess catalytic characteristicsfor oxidizing CO.

FIG. 5 schematically shows a system used for chromatographic analysis ofcatalytic characteristics of samples.

FIG. 6 shows an SEM of a composite particle of the invention in whichnanoporous titania particles and aggregates thereof are coated ontocarbon host particles.

FIG. 7 shows an SEM of a composite particle of the invention in whichnanoporous titania particles and aggregates thereof are coated ontocarbon host particles.

FIG. 8 shows an SEM of a composite catalyst of the invention in whichnanoporous titania particles and aggregates thereof are coated ontocarbon host particles, wherein gold has been deposited on the titaniausing PVD techniques.

FIG. 9 is a graph demonstrating the effectiveness by which the catalystsof Examples 4 through 7 catalyze CO.

FIG. 10 is a graph demonstrating the effect of step change in CO inletconcentration under conditions itemized in Example 1.

FIG. 11 is a graph demonstrating the results of Example 2.

FIG. 12 is a graph demonstrating the results of Example 3.

FIG. 13 is a graph demonstrating the results of Comparative Example A.

FIG. 14 is a graph demonstrating the results of Example 9.

DETAILED DESCRIPTION

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention. While the present invention will be described in the specificcontext of gold-based catalyst systems, the principles of the inventionare applicable to other catalyst systems as well.

In the practice of the present invention, catalytically active gold maybe deposited on the desired support(s) in any fashion, but preferably isdeposited using physical vapor deposition. Physical vapor depositionrefers to the physical transfer of gold from a gold-containing source ortarget to the support. Physical vapor deposition may be viewed asinvolving atom-by-atom deposition although in actual practice, the goldmay be transferred as extremely fine bodies constituting more than oneatom per body. Once at the surface, the gold may interact with thesurface physically, chemically, ionically, and/or otherwise. Usingphysical vapor deposition methodologies to deposit nanoscale gold onactivating, nano-porous support media makes the synthesis ofcatalytically active gold dramatically easier and opens the door tosignificant improvements associated with developing, making, and usinggold-based, catalytic systems.

Some modes of practice, particularly those using lower amounts ofdeposited gold, involve depositing gold via PVD only after the supportmedia has been impregnated with one or more activating agents and/orother impregnant(s), dried, and optionally calcined or otherwise heattreated. This greatly expands the range of activating agents that can beused in combination with a catalytically active metal. We can useingredients that would otherwise react or be too soluble in solutionwhen wet methods are used to deposit gold. For instance, the process ofthe invention can deposit gold or other metals onto media comprisingvery basic or water-soluble materials. This has opened the door totesting and using water soluble, metal salts as activating agentsinasmuch as these are not washed away when gold is subsequentlydeposited via PVD. It would not be very practical to attempt to use suchsalts as activating agents when gold is impregnated onto the supportmedia via solution processing, inasmuch as the gold solutions could washaway the water soluble material and/or be chemically incompatible withthe activating agents (e.g., gold solutions tend to be strongly acidic,e.g., HAuCl₄).

We have observed that a catalytically active metal such as gold isactive right away when deposited via PVD. There is no need to heat treatthe system after gold deposition as is the case with some othermethodologies, although such heat treating may be practiced if desired.Additionally, the gold is highly active catalytically for relativelylong periods with respect to CO oxidation, even though it tends to bedeposited only proximal to the support media surface when using PVD todeposit the gold. The catalyst systems also are effective in humidenvironments and work over a wide temperature range, including roomtemperature (e.g., about 22° C. to about 27° C.) and much cooler (e.g.,less than 5° C.).

The physical vapor deposition process is very clean in the sense thatthere are no impurities introduced into the system as in the case of thesolution state processes. In particular, the process may bechloride-free and thus there is no need for washing steps to removechloride or other undesirable ions, molecules or reaction by-products,as is the case in most solution state deposition processes.

By using this process, very low levels of metal are required for highactivity. While most research in this area uses at least 1% by weightgold to achieve activity, and often times much more than 1 weight % goldto achieve high activity, in this work we have achieved very highactivity at 0.15% by weight gold or lower. This reduction in the amountof precious metal required for high activity provides a very substantialcost savings. Yet, other embodiments of the present invention, such asguest/host composite systems, provide high performance using higherlevels of gold, e.g., 0.5% to 5% by weight gold.

This process results in a very uniform product with respect to preciousmetal concentration per particle and metal nanoparticle size and sizedistribution. TEM studies have shown that our process can deposit goldin a form including discrete nanoparticles and small clusters or in amore continuous thin film depending on what is desired. In general, itis desired to include gold in nanoparticle/small gold cluster form.

This catalyst preparation method can deposit catalyst metals uniformlyon non-uniform or non-homogeneous surfaces. This is not true for thesolution state deposition processes that tend to favor deposition on thesurfaces having a charge opposite to the depositing metal ion, leavingthe other surfaces uncoated or at best weakly coated.

In addition to gold, the PVD process can be used to deposit other metalssimultaneously or sequentially or to deposit mixtures of metals by usingpoly-phasic targets so that catalyst particles can be formed thatcomprise polyphasic nanoparticles, e.g., nanoparticles comprising atomicmixtures of say M₁ and M₂ (where M₁ and M₂ represent different metals),or that have combinations of metal nanoparticles for multi-functioncatalysts, e.g., nanoparticle mixtures comprising mixtures of discreteM₁ particles and discrete M₂ particles. In this fashion, catalystparticles can be prepared that can catalyze more than one reaction andthese functions can be carried out simultaneously in practice. Thus, forinstance, a catalyst particle can be prepared that will oxidize CO whileat the same time oxidize SO₂ efficiently.

The PVD approach can efficiently deposit catalytically active metals ona wider range of support media, e.g., not only particles but alsohoneycombs, fibers, scrims, fabrics, paper, and the like. While fiberscan be coated in the solution coating processes, the shear used to pulpand disperse the fibers in those processes generally results in dustformation and in inefficient coating due to the abrasion of the fibersduring the coating process. PVD techniques are much more facile in thisregard.

This PVD process allows catalytically active gold to be easily depositedonto supports containing carbon as well as on other oxidativelysensitive substrates. In the processes known in the art that require aheating step to affix and activate the catalyst particles, carbon in thepresence of an oxidizing environment cannot adequately withstand theelevated temperatures that are often used. Thus, the carbon particleshad to be treated in a reducing atmosphere since they would be attackedby oxygen during this heating step. Such a reducing step may undesirablyreduce other catalyst constituents (e.g., as in the case of iron oxidesupported on carbon or in porous carbon). In the instant invention,carbon particles and other non-oxide particles can be coated withcatalyst nanoparticles and no heating step or post reduction isrequired. In this manner, high surface area carbon can be renderedcatalytic for CO oxidation without losing the adsorptive properties ofthe porous carbon for the removal of other impurities from a gas stream.

The PVD approach can be used to coat very fine particles with catalystwherein the fines are already coated on a larger host material.Alternatively, the PVD approach can be used to coat catalyst onto veryfine particles before the fine particles are coated onto a secondgranular phase or other host or are thereafter formed into a porousgranule. With either approach, the resultant composite provides high COoxidation activity with low backpressure during use.

Physical vapor deposition preferably occurs under temperature and vacuumconditions in which the gold is very mobile. Consequently, the gold isquite mobile and will tend to migrate on the surface of the substrateuntil immobilized in some fashion, e.g., by adhering to a site on orvery near the support surface. It is believed that sites of adhering caninclude defects such as surface vacancies, structural discontinuitiessuch as steps and dislocations, interfacial boundaries between phases orcrystals or other gold species such as small gold clusters. It is adistinct advantage of the invention that the deposited gold isimmobilized effectively in a manner in which the gold retains a highlevel of catalytic activity. This is contrasted to those conventionalmethodologies in which the gold accumulates into such large bodies thatcatalytic activity is unduly compromised or even lost.

There are different approaches for carrying out physical vapordeposition. Representative approaches include sputter deposition,evaporation, and cathodic arc deposition. Any of these or other PVDapproaches may be used, although the nature of the PVD technique usedcan impact catalytic activity. For instance, the energy of the physicalvapor deposition technique used can impact the mobility, and hencetendency to accumulate, of the deposited gold. Higher energy tends tocorrespond to an increased tendency of the gold to accumulate. Increasedaccumulation, in turn, tends to reduce catalytic activity. Generally,the energy of the depositing species is lowest for evaporation, higherfor sputter deposition (which may include some ion content in which asmall fraction of the impinging metal species are ionized), and highestfor cathodic arc (which may be several tens of percents of ion content).Accordingly, if a particular PVD technique yields deposited gold that ismore mobile than might be desired, it may be useful to use a PVDtechnique of lesser energy instead.

Physical vapor deposition generally is a line of sight/surface coatingtechnique between the gold source and the support. This means that onlythe exposed, outer surfaces of the support, but not the inner pores wellwithin the substrate, are directly coated. Inner surfaces not in adirect line of sight with the source will tend not to be directly coatedwith gold. However, we have found by TEM analysis that after depositionon the surface of a porous substrate, the gold atoms can migrate bydiffusion or other mechanism some moderate distance into the catalystsurface to provide nano-particles and gold clusters in the substratepores in the region immediately adjacent to the surface before beingimmobilized. The average penetration into the porous substrates can beup to 50 nanometers in depth or sometimes greater, such as up to about70 to about 90 nm in depth. In general though, the penetration depth isless than 50 nm and can be less than 30 nm. The gold penetration is veryshallow compared to the typical support size.

The total thickness of the gold, or C_(t), is equal to the goldpenetration depth plus the thickness of the gold that is deposited onthe surface of the substrate and that has not penetrated by diffusion.This total thickness is in general less than 50 nm and can often be lessthan 30 nm or even less than 20 nm. On materials having surface poreswhose depth is greater than about 10 nm to 20 nm, the total goldthickness can appear to be greater than 50 nm since the gold layerfollows the contours of the surface and the actual surface contour isreflected by the pore structure that it possesses. It is most preferredthat the active gold species be collected on the outermost portion ofthe catalyst particle since this is the surface of the catalyst thatinteracts most readily with gaseous reactants.

The thickness of the gold shell region relative to the catalyst supportparticle size is quantified by the formulaPDR=C _(t)/USTwherein PDR is the penetration depth ratio, UST is the underlyingsupport thickness or particle size and C_(t) is the total thickness ofthe gold, as defined above. The underlying support thickness representsthe size of the support as measured perpendicular to the catalystsurface and is usually indicative of particle size. The underlyingsupport thickness may be determined by microscopic methods includingoptical microscopy or scanning electron microscopy. The value for C_(t)may be determined by transmission electron microscopy in the case ofthin films and high resolution scanning electron microscopy in the caseof thicker films. The total thickness C_(t) is very easily discernedfrom visual inspection of TEM data. Because of the uniformity by whichgold is coated, a single representative TEM picture can be effective tocharacterize the coating. In practice, a sample may be effectivelycharacterized via examination of a number of TEM pictures of catalystsurface cross-sections (vida infra) In preferred embodiments, PDR is inthe range of from about 1×10⁻⁹ to 0.1, preferably 1×10⁻⁶ to 1×10⁻⁴,indicating that the gold shell region is very thin indeed relative tototal support thickness. As noted above, this generally corresponds to apenetration depth on the order of up to about 50 nm, preferably about 30nm on preferred supports.

Characterization of the surface region and the gold bodies isaccomplished using transmission electron microscopy as is well-known inthe catalyst art. One method suitable for characterizing the catalyticsurfaces is as follows: the catalyst particles are embedded in 3MScotchcast™ Electrical Resin #5 (epoxy; 3M Company, St. Paul, Minn.) indisposable embedding capsules; resin is allowed to cure at roomtemperature for 24 hours.

For each sample, a random, embedded granule is trimmed (with a stainlesssteel razor blade previously cleaned with isopropyl alcohol) down to themiddle surface region of the granule such that most of the granule iscut away on one side, leaving epoxy on the other side. A smalltrapezoid-shaped face (less than a half millimeter on a side) isselected and trimmed such that the epoxy/granule interface is leftintact. The long direction of this interface is also the cuttingdirection. A Leica Ultracut UCT microtome (Leica Microsystems Inc.,Bannockburn, Ill.) is used to cross-section the face. The face is firstaligned such that the granule surface was perpendicular to the knifeedge. Sections approximately 70 nm thick are cut at a speed of 0.08mm/second. These sections are separated by floating onto deionized waterand collected using a microtomy hair tool and picked up using a “PerfectLoop” (loop distributed by Electron Microscopy Sciences, FortWashington, Pa.). Samples are transferred via this loop to a 3 mmdiameter, 300 mesh copper TEM grid with carbon/formvar lacey substrate.The regions of interest (intact, cleanly cut specimens showing theinterfacial region) that lie over the holes in the substrate are imagedand analyzed.

Images are taken at various magnifications (50,000× and 100,000×) in aHitachi H-9000 transmission electron microscope (TEM; Hitachi HighTechnologies America, Pleasanton, Calif.) at 300 KV accelerating voltageusing a Gatan CCD camera (Gatan Inc., Warrenton, Pa.) and DigitalMicrograph software. Representative regions (regions selected whereinthe interface of the catalytic surface is clearly displayed in a fashionperpendicular to the surface of the sample) are imaged. Calibratedmarkers and sample identifications are placed on each image. Numerous(>10) interfacial regions are examined.

An example of a TEM image of a cross-section of a representativecatalyst surface of the present invention (material of example 3 ofAssignee's Co-Pending Application cited above) is shown in FIG. 1. Thegold nanoparticles can be seen to be both on the surface of the supportand in the sub-surface region of the support. The region containing thegold nanoparticles is very thin and the gold deposition can be seen tofollow the contours of the surface of the support.

As a consequence of line of sight coating, the resultant catalyticallyactive material of the invention from one perspective may be viewed asnanoporous catalytic supports having relatively thin shells ofdiscontinuous, catalytic gold on and proximal to their outer surfaces.That is, a resultant catalytically active material comprises a gold-richshell region proximal to the surface and an interior region comprisingnegligible gold. In preferred embodiments, this gold-rich shell regioncomprises small (generally less than 10 nm, most preferably less than 5nm), discrete gold bodies.

The inventive approach of forming a catalytically active shell regiononly on the surface of a nanoporous support is contrary to conventionalwisdom when developing new catalytic material, and, therefore, the factthat the resultant material is so catalytically active is quitesurprising. Specifically, the present invention puts catalyticfunctionality only near the surface of a highly porous support. Interiorporosity is purposely unused. From a conventional perspective, it seemspointless to underutilize a nanoporous support in this manner. Knowingthat catalytically active metal is to be deposited only at the supportsurface, the conventional bias might have been to use a nonporoussubstrate when depositing catalytically active gold onto a support. Thisis especially the case when PVD is not able to access the interior ofthe porous support in any event. The present invention overcomes thisbias through the combined appreciation that (1) gold mobility is highlyrestricted on the surface of nanoporous supports, and (2) gold is stillcatalytically active even at very low weight loadings resulting from thesurface coating approach. Consequently, using such supports is highlyand uniquely beneficial in the context of depositing gold onto thesurface region of a nanoporous support even though full catalyticcapacity of the support is not utilized. For this reason, catalyticallyactive gold is readily formed on composite supports (described furtherbelow) in which nanoporous “guest” particles are deposited onto “host”material, which itself may or may not be nanoporous.

Generally, physical vapor deposition preferably is performed while thesupport to be treated is being well-mixed (e.g., tumbled, fluidized, orthe like) to help ensure that particle surfaces are adequately treated.Methods of tumbling particles for deposition by PVD are summarized inU.S. Pat. No. 4,618,525. For methods specifically directed at catalystssee Wise: “High Dispersion Platinum Catalyst by RF Sputtering,” Journalof Catalysis, Vol. 83, pages 477-479 (1983) and Cairns et al U.S. Pat.No. 4,046,712. More preferably, the support is both tumbled or otherwisefluidized as well as comminuted (e.g., ground or milled to some degree)during at least a portion of the PVD process. This provides a degree ofmechanical abrasion of the surface of the particles and generation ofsome fines during gold deposition. Our data suggests that catalyticperformance is enhanced when deposition is carried out with comminution.It is our belief that these processes, i.e., the generation of fines andthe mechanical interaction of the grits with each other, increases theactivity of the resulting catalyst materials. While not wishing to bebound by theory, we believe that the fines provide higher surface areafor higher activity. Fresh surface areas of the support are alsoexposed, and this might also enhance performance.

The impact of such comminution upon the resultant surfacecharacteristics of the catalyst system was studied via TEM analysis. Inthe case of the gold on carbon containing the activating agents of thepresent invention, the TEMs reveal the presence of a unique, two phasestructure believed to comprise nanoparticles and clusters of gold andcarbonaceous material on the surface of the gold-coated particles. Thisnano-composite of gold/activation agent and carbon seems to possess avery high activity for catalysis of CO oxidation.

Such comminution, however, may not be desired in those embodiments inwhich gold is deposited onto composite supports fabricated fromingredients comprising guest and host material. Grinding tends to reducethe activity of these composite-based catalyst systems. In the case ofcomposite structures containing titania guest material coated ontocarbon host material, and without wishing to be bound, the reducedactivity may be due to fines of carbon being generated and deposited onthe titania. This tends to reduce the amount of gold/titania interfacesassociated with higher activity.

An apparatus 10 for carrying out the preferred PVD process is shown inFIGS. 2 and 3. The apparatus 10 includes a housing 12 defining a vacuumchamber 14 containing a particle agitator 16. The housing 12, which maybe made from an aluminum alloy if desired, is a vertically orientedhollow cylinder (45 cm high and 50 cm in diameter). The base 18 containsa port 20 for a high vacuum gate valve 22 followed by a six-inchdiffusion pump 24 as well as a support 26 for the particle agitator 16.The chamber 14 is capable of being evacuated to background pressures inthe range of 10⁻⁶ torr.

The top of the housing 12 includes a demountable, rubber L-gasket sealedplate 28 that is fitted with an external mount three-inch diameter dcmagnetron sputter deposition source 30 (a US Gun II, US, INC., San Jose,Calif.). Into the source 30 is fastened a gold sputter target 32 (7.6 cm(3.0 inch) diameter×0.48 cm ( 3/16 inch) thick). The sputter source 30is powered by an MDX-10 Magnetron Drive (Advanced Energy Industries,Inc, Fort Collins, Colo.) fitted with an arc suppressing Sparc-le 20(Advanced Energy Industries, Inc, Fort Collins, Colo.).

The particle agitator 16 is a hollow cylinder (12 cm long×9.5 cmdiameter horizontal) with a rectangular opening 34 (6.5 cm×7.5 cm) inthe top 36. The opening 34 is positioned 7 cm directly below the surface36 of the gold sputter target 32 so that sputtered gold atoms can enterthe agitator volume 38. The agitator 16 is fitted with a shaft 40aligned with its axis. The shaft 40 has a rectangular cross section (1cm×1 cm) to which are bolted four rectangular blades 42 which form anagitation mechanism or paddle wheel for the support particles beingtumbled. The blades 42 each contain two holes 44 (2 cm diameter) topromote communication between the particle volumes contained in each ofthe four quadrants formed by the blades 42 and agitator cylinder 16. Thedimensions of the blades 42 are selected to give side and end gapdistances of either 2.7 mm or 1.7 mm with the agitator walls 48.Preferred modes of use of this apparatus are described below in theexamples.

Physical vapor deposition may be carried out at any desiredtemperature(s) over a very wide range. However, the deposited gold maybe more catalytically active if the gold is deposited at relatively lowtemperatures, e.g., at a temperature below about 150° C., preferablybelow about 50° C., more preferably at ambient temperature (e.g., about20° C. to about 27° C.) or less. Operating under ambient conditions ispreferred as being effective and economical since no heating or chillingrequirements are involved during the deposition

While not wishing to be bound by theory, it is believed that thedeposition at lower temperatures yields more catalytically active goldfor at least two reasons. First, lower temperatures yield gold with moredefects in terms of geometrical size and/or shape (angularities, kinks,steps, etc.). Such defects are believed to play a role in many catalyticprocesses (see Z. P. Liu and P. Hu, J. Am. Chem. Soc., 2003, 125, 1958).On the other hand, deposition at higher temperatures tends to yield goldthat has a more organized and defect-free crystal structure and hence isless active. Additionally, deposition temperature can also impact goldmobility. Gold tends to be more mobile at higher temperatures and hencemore likely to accumulate and lose catalytic activity.

The present invention provides catalytically active gold on the desiredsupport(s) to form heterogeneous catalytic systems of the presentinvention. Gold is widely known as a noble, relatively inert metal witha yellowish color. However, the characteristics of gold changedramatically in nanoscale regimes, where gold becomes highlycatalytically active. The high reactivity of gold catalyst in comparisonwith other metal catalysts is illustrated by reactions such as oxidationof CO under ambient conditions and reduction of NO, as well asepoxidation and hydrochlorination of unsaturated hydrocarbons.

In preferred embodiments, catalytically active gold may be identified byone or more requisite characteristics including size, color, and/orelectrical characteristics. Generally, if a gold sample has one or moreof these requisite characteristics, and preferably two or more of thesecharacteristics, it will be deemed to be catalytically active in thepractice of the present invention. Nanoscale size is a key requisiteassociated with catalytically active gold in that the catalytic activityof gold to a large degree is a function of whether the gold sample has athickness dimension in the nanoscale regime (e.g., particle diameter,fiber diameter, film thickness, or the like). Bodies (also referred toas clusters in the literature) having smaller dimensions tend to be morecatalytically active. As size increases, catalytic characteristics falloff rapidly. Accordingly, preferred embodiments of catalytically activegold may have a nanoscale size over a wide range, with smaller sizesmore preferred when higher activity is desired. As general guidelines,catalytically active gold has particle or cluster dimensions in therange of from about 0.5 nm to about 50 nm, preferably about 1 nm toabout 10 nm. Preferably, the gold has a size of no more than about 2 nmto about 5 nm in any dimension. The technical literature reports thatcatalytic activity may be a maximum at sizes in the range of from about2 nm to about 3 nm. The size of the individual gold nanoparticles can bedetermined by TEM analysis as is well known in the art and as isdescribed herein.

In terms of color, gold in larger scale size regimes has a yellowishcolor. However, in the nanoscale size regimes in which gold iscatalytically active, the color of gold becomes a reddish-pink and thenpurplish-blue when viewed under white light, although very smallclusters of gold and gold surface species can be colorless. Suchcolorless species can be quite catalytic, and the presence of suchcolorless species is usually accompanied by some colored nanoparticlesof gold. Consequently, determining if the color of a gold sampleincludes a noticeable reddish-pink to purplish-blue component and/or iscolorless indicates that it is possible that the sample is catalyticallyactive.

The amount of catalytically active gold provided on a support can varyover a wide range. However, from a practical perspective, it is helpfulto consider and balance a number of factors when choosing a desiredweight loading. For instance, catalytically active gold is highly activewhen provided on nanoporous supports in accordance with the practice ofthe present invention. Thus, only very low weight loadings are needed toachieve good catalytic performance. This is fortunate, because gold isexpensive. For economic reasons, therefore, it would be desirable not touse more gold than is reasonably needed to achieve the desired degree ofcatalytic activity. Additionally, because nanoscale gold is highlymobile when deposited using PVD, catalytic activity may be compromisedif too much gold is used due to accumulation of the gold into largebodies. With such factors in mind, and as general guidelines, the weightloading of gold on the support preferably is in the range of 0.005 to 5weight %, preferably 0.005 to 2 weight %, and most preferably from 0.005to 1.5 weight % based upon the total weight of the support and the gold.When the support is a composite of 2 or more constituents, e.g., acomposite formed by providing a plurality of one or more kinds of guestparticles on one or more kinds of host particles, the total weight ofthe support refers to the total weight of the resultant composite.

Depositing catalytically active gold onto a support is very compatiblewith PVD techniques. Gold naturally sputters to form catalyticallyactive, nanoscale particles and clusters onto the nanoporous supportsurface. It is believed that the gold is deposited mainly in elementalform, although other oxidation states may be present. Although gold ismobile and will tend to accumulate in low energy sites of the surface,the nanoporous characteristics of the support and the preferred use ofactivating agents in the practice of the present invention help toimmobilize the gold, helping to keep the deposited gold clustersisolated and preferably discontinuous. This helps to preserve catalyticactivity that might be otherwise compromised if the gold were toaccumulate into larger sized bodies. As an alternative, very thin, goldfilms of nanoscale thickness may also be formed over some or all of thesupport surface if desired, keeping in mind that catalytic activitydecreases with increasing film thickness. Even though such films may beformed with catalytic activity, discontinuous, isolated gold clusterstend to be much more catalytically active and are preferred in mostapplications.

Optionally, the heterogeneous catalyst system may be thermally treatedafter gold deposition if desired. Some conventional methods may requiresuch thermal treatment in order to render the gold catalytically active.However, gold deposited in accordance with the present invention ishighly active as deposited without any need for a thermal treatment.Indeed, such gold can very effectively catalytically oxidize CO to formCO₂ at room temperature or even much cooler. Additionally, dependingupon factors such as the nature of the support, the activating agents,the amount of gold, or the like, catalytic activity can be compromisedto some degree if thermally treated at too high a temperature. Indeed,for some modes of practice in which the heterogeneous catalyst system isintended to be used in a heated environment, e.g., an environment havinga temperature higher than about 200° C., the catalytic activity of thesystem should be confirmed at those temperatures.

It is also believed that low-coordination gold in catalyticnanoparticles is beneficial. Low coordination gold refers to Au_(n) forwhich n on average is in the range of 1 to 100, preferably about 2 to20. Without wishing to be bound by theory, we propose that the catalyticactivity of the very small clusters of gold is associated at least tosome degree with low-coordination defects, and that these defects areable to provide sites for storing charges which may be transferred fromunderlying supports and/or other sources. Accordingly, with such defectsand mechanism in mind, it is preferred that heterogeneous catalysts ofthe invention include one or more of the following features: (a) Thegold and hence the defects are located mainly on the surface of theunderlying support; (b) The average value for n is greater than about 2;and (c) as much as is practically possible, gold clusters are isolatedbut nonetheless close to each other (within a distance of about 1 nm toabout 2 nm or less). While such features may be associated with smallersized gold clusters, it is possible that such characteristics may befound mainly at steps or edges of larger clusters.

In addition to gold, one or more other catalysts could also be providedon the same supports and/or on other supports intermixed with thegold-containing supports. Examples include one or more of silver,palladium, platinum, rhodium, ruthenium, osmium, copper, iridium, or thelike. If used, these may be co-deposited onto the support from a targetsource that is the same or different than the gold source target.Alternatively, such catalysts may be provided on the support eitherbefore or after the gold. Other catalysts requiring a thermal treatmentfor activation advantageously may be applied onto the support and heattreated before the gold is deposited. In certain cases catalysts such asRh, Pd and Pt can be deposited according to the present invention andutilized as catalysts without the presence of gold.

In the practice of the present invention, catalytically active gold isdeposited onto one or more supports that are nanoporous. We have foundthat surface deposition/coating of catalytically active metal onto thenanoscale topography of nanoporous media provides catalyst systems withexcellent performance. In the case of gold, for example, it appears thatthese nanoscale features help to immobilize the gold, preventing goldaccumulation that might otherwise result in a loss of performance.

Nanopores can be observed and nanopore size can be measured viatransmission electron microscopy. The nanoporous nature of a support mayalso be characterized by a technique such as described in ASTM StandardPractice D 4641-94 in which nitrogen desorption isotherms are used tocalculate the pore size distribution of catalysts and catalyst supportsin the range from about 1.5 to 100 nm. Nanoporous means that the totalnanoporous capacity for pores in the size range of 1 to 10 nm is greaterthan 20% (i.e., greater than about 0.20 using the formula below) of thetotal pore volume of the support material in the range from 1 to 100 nmas calculated using the following formula with data obtained from ASTMD4641-94, the entirety of which is incorporated herein by reference:

${NPC} = \frac{{CPv}_{1} - {CPv}_{10}}{{CPv}_{1} - {CPv}_{100}}$wherein NPC refers to the nanoporous capacity; CPv_(n) refers to thecumulative pore volume at pore radius n in cm³/g; and n is the poreradius in nanometers.

The nanoporous characteristic of the support helps to immobilize goldclusters on the support surface. This stabilization of the very smallgold particles and clusters is evidenced by both the direct observationof smaller particles of gold in TEM studies of materials possessingnanoporous surfaces and in higher catalytic activity as measured by theability of the catalyst to convert CO to CO₂ in the presence of air.

The nanoporous characteristic of the support helps to immobilize goldclusters on the support surface. This stabilization of the very smallgold particles and clusters is evidenced by both the direct observationof smaller particles of gold in TEM studies of materials possessingnanoporous surfaces and in higher catalytic activity as measured by theability of the catalyst to convert CO to CO₂ in the presence of air.Advantageously, gold is also readily deposited onto nanoporous supportsusing PVD in a catalytically active state without requiring additionalthermal or other treatment for activation. In addition to nanoporosity,the substrate particles optionally may further have microporous,mesoporous, and/or macroporous characteristics as such are defined inapplicable provisions of IUPAC Compendium of Chemical Technology, 2dedition (1997). A typical population of activated carbon or aluminasupport particles will tend to include a combination of nanoporous,microporous, mesoporous, and macroporous properties.

It is important to note that the support materials only need benanoporous in the exterior surface region of the support at a depthequal to or greater than the penetration depth of the gold atoms in thepresent invention. Thus, the present invention includes methods wherebynormally low surface area, non-nanoporous materials can be made topossess exterior surfaces characterized by nanoporosity. These methodsinclude adsorption of nanoporous materials such as gels and nanoparticlesize colloids on the surface of a larger, host material to form acomposite with the desired nanoporosity; hydrolysis of metal alkoxidesor metal salts on the surface of a material to form the nanoporousmaterials; and oxidation of a thin coating of metal, e.g., aluminum,titanium, tin, antimony or the like, on the surface of a material toform a nanoporous material. In the latter case, the thin metal films canbe deposited by physical vapor methods and the oxidation can be carriedout by dry or moist air to produce a nanoparticle film on the substrate.

The support(s) may be used in various shapes or combinations such as,for example, powder, particle, pellet, granule, extrudate, fiber, shell,honeycomb, plate, or the like. The particles can be regular in shape,irregular, dendritic, dendrite-free, or the like. Preferred supports areparticulate in nature or powders.

Particulate embodiments of support media may have any of a wide range ofsizes. Support particle size generally may be expressed in terms of amesh size. A typical expression for mesh size is given by “a×b”, wherein“a” refers to a mesh density through which substantially all of theparticles would fall through, and “b” refers to a mesh density that issufficiently high so as to retain substantially all of the particles.For example, a mesh size of 12×30 means that substantially all of theparticles would fall through a mesh having a mesh density of 12 wiresper inch, and substantially all of the particles would be retained by amesh density having a density of 30 wires per inch. Support particlescharacterized by a mesh size of 12×30 would include a population ofparticles having a diameter in the range from about 0.5 mm to about 1.5mm.

Selecting an appropriate mesh size for the substrate particles involvesbalancing density and catalytic rate against air flow resistance.Generally, a finer mesh size (i.e., smaller particles) tends to providenot only greater catalytic rate and filter capacity, but also higher airflow resistance. Balancing these concerns, “a” is typically in the rangeof 8 to 12 and “b” is typically 20 to about 40 with the proviso that thedifference between a and b is generally in the range from about 8 toabout 30. Specific mesh sizes found to be suitable in the practice ofthe present invention include 12×20, 12×30, and 12×40.

In addition to nanoporosity, support media of the present inventionpreferably further include one or more additional characteristics. Forinstance, preferred embodiments of the support media are characterizedby multiphasic, e.g., biphasic, surfaces. Multiphasic means that thesurface has more than one phase. Our data show that catalytic activityis enhanced when gold is deposited onto a multiphasic surface. While notwishing to be bound, it is believed that the resultant phase boundarieson the surface appear to help stabilize gold. TEM studies as describedherein and as is well known in the art can be used to assess whether asurface is biphasic. It is believed that these phase boundaries are veryfinely dispersed at the nanoscale, helping to make the boundarieseffective for immobilizing gold.

A wide variety of materials may serve as suitable supports in thepractice of the present invention. Representative examples includecarbonaceous materials, silicaceous materials (such as silica), metalcompounds such as metal oxides or sulfides, combinations of these, andthe like. Representative metal oxides (or sulfides) include oxides (orsulfides) of one or more of magnesium, aluminum, titanium, vanadium,chromium, manganese, cobalt, nickel, copper, zinc, gallium, germanium,strontium, yttrium, zirconium, niobium, molybdenum, technetium,ruthenium, rhodium, palladium, silver, cadmium, indium, iron, tin,antimony, barium, lanthanum, hafnium, thallium, tungsten, rhenium,osmium, iridium, and platinum.

Examples of carbonaceous substances include activated carbon andgraphite. Suitable activated carbon particles may be derived from a widevariety of source(s) including coal, coconut, peat, any activatedcarbon(s) from any source(s), combinations of at least two of these,and/or the like.

Preferred embodiments of support media may be selected from aluminumoxides, titania, titania-alumina, activated carbon, binary oxides suchas hopcalite (CuMnO₂), molecular sieves, and/or the like. Of these,alumina, titania and activated carbon are particularly preferred supportmaterials. Activated carbon, titania and alumina are found in formshaving nanoporosity and therefore, these forms are preferred supportmaterials. Activated carbon is advantageous because in addition toproviding a support for catalytic activity, the carbon also functions asan absorbent for noxious gases. Additional impregnants that augmentfiltering capabilities are also easily incorporated into carbonaceousmaterial in accordance with conventional practices (described furtherbelow). Activated alumina also is a preferred support material, as it isvery robust to aging and heat. Generally, catalyst systems of thepresent invention are advantageously made from ingredients comprising analumina support when the catalyst system will be used at elevatedtemperature. Otherwise, supports comprising activated carbon arepreferred in many embodiments, as these generally have longer servicelives.

A particularly preferred support is a composite that may be prepared byadsorbing or adhering fine (less than 100 micrometers, preferably lessthan 50 micrometers and most preferably less than 10 micrometer)nanoporous particles onto larger material, fibers, honeycomb material,combinations of these, and the like. The relatively fine material isreferred to herein as “guest” material, while the relatively largersupport material is referred to herein as “host” material. As onealternative, catalytically active gold may be deposited onto the guestmaterial before the guest material is combined with the host material.As another alternative, catalytically active gold may be deposited ontothe resultant composite material during or after the composite materialis formed.

This guest/host composite structure provides dramatically higher totalexterior surface area while retaining the desirable gas passingcharacteristics, i.e., low pressure drop, of a coarser particle. Inaddition, by using nanoporous, smaller particles in constructing thesecomposite particles, inexpensive, non-nanoporous, coarser particles canbe used. Thus, very inexpensive, highly active catalyst particles can beprepared since the bulk of the volume of a catalyst bed is taken up bythe inexpensive, underlying, coarser particles.

A variety of methods generally may be used to construct the compositesupport media. In one method, nanoporous, guest particles are admixedwith one or more adhesion agents in solution and then this mixture iscombined with coarser host particles. If the coarser particle is porous,the small particle-adhesion agent solution mixture can be introduced byincipient wetting of the porous larger particle. If the larger particleis not porous, the small particle-adhesion agent solution mixture can beadmixed with the coarser particles and the solution liquid can beremoved either concurrent with the mixing or subsequent to the mixing.In either case, after combining the nanoporous, small particle sizematerial, the adhesion agent and the coarser particles and removing theliquid from the solution, the mixture is dried and optionally calcinedor otherwise heat treated to provide a composite particle having thesmaller, nanoporous particles adhered on the surface of a coarserparticle. The calcining temperature is selected to be below thetemperature at which the nanoporous particles lose porosity. Generallythe calcining temperature will be in the range of about 200° C. to about800° C. In general, a low temperature is preferred. The sample is heatedsufficiently to generate a bond between the adhesion agent and theparticles but not high enough to significantly alter the nanoporousnature of the coating. The adhesion agent generally is included at anamount of 0.1 to about 50 parts by weight based upon 100 parts by weightof the guest material. Examples of adhesion agents include basic metalsalts, partially hydrolyzed metal complexes such as partially hydrolyzedalkoxides, hydrous metal oxy-hydroxide nanoparticles, and other metalsalts. Samples containing carbon, though, generally are heated at moremoderate temperatures, e.g., 120° C. to 140° C. As another constructionmethod for making composite support media, guest particles can beadhered to the host particles using partially hydrolyzed alkoxidesolutions, basic metal salt solutions, or nanoparticle sized colloidalmetal oxides and oxy-hydroxides as an adhesion agent. Partiallyhydrolyzed alkoxide solutions are prepared as is well known in thesol-gel art. Useful metal alkoxides include alkoxides of titanium,aluminum, silicon, tin, vanadium and admixtures of these alkoxides.Basic metal salts include nitrate and carboxylate salts of titanium andaluminum. Nanoparticle size colloidal materials include colloids ofoxides and oxy-hydroxides of aluminum, titanium and oxides of silicon,tin, and vanadium.

As an alternative construction method, guest-host composites can beprepared by physically mixing guest and host materials. This can occurby techniques involving mechanical and/or electrostatic mixing. As aconsequence of this mixing, the guest and host components tend to becomeassociated into desired ordered mixtures in which guest materialsubstantially uniformly coats or is otherwise associated with thesurfaces of the host material. Optionally, one or more liquidingredients may be included in the ingredients used to make an orderedmixture, although dry blending with little or no solvent can providesuitable composites. Although not wishing to be bound, it is believedthat the guest material may physically, chemically, and/orelectrostatically interact with the host material to form the orderedmixture. Ordered mixtures and methods of making such mixtures has beendescribed in Pfeffer et al., “Synthesis of engineered Particulates withTailored Properties Using Dry Particle Coating”, Powder Technology 117(2001) 40-67; and Hersey, “Ordered Mixing: A New Concept in PowderMixing Practice”, Powder Technology, 11 (1975) 41-44, each of which isincorporated herein by reference.

Preferably, one or more kinds of nanoporous guest particles are presentin nanoparticulate form and independently may have a median particlesize in the range of from about 3 nm to about 35 nm, more preferablyabout 3 nm to about 15 nm, and most preferably about 3 nm to about 8 nm.The guest particles preferably have a high surface area as measured byBET. The surface area is preferably greater than about 50 m²/g, morepreferably greater than about 150 m²/g. and most preferably greater thanabout 300 m²/g.

The guest particles may be present in the form of nanoporous aggregatesof nanoparticles. These nanoporous aggregate particles may have a medianparticle size in the range of from about 0.2 micrometers to about 3micrometers, more preferably in the range of about 0.2 micrometers toabout 1.5 micrometers, and most preferably in the range of about 0.2micrometers to about 1.0 micrometers.

The guest particles and/or aggregated guest particles provide ananoporous, exposed, high surface area coating on the host particles forthe vapor deposition of gold.

As shown in FIGS. 6 and 7, the SEM images of guest/host embodimentscomprising titania particles and titania aggregates coated as guestmaterial onto carbon host particles show porosity on multiple levels.FIG. 8 shows gold deposited on such particles using PVD techniques todeposit the gold.

In embodiments in which the host material includes particulateconstituents, the one or more kinds of host particles are larger thanthe guest material being used and typically independently may have amedian particle size in the range of from 3 micrometers to about 1000micrometers, more preferably in the range of about 5 micrometers toabout 500 micrometers. However, larger host particles may be used insome applications. Within such ranges, it is also desirable that therelative sizes of the host and guest particles are suitable for formingan ordered mixture. Thus, it is preferred that the ratio of the volumeaverage particle size of the host particles to the guest particles isgreater than about 3:1, more preferably greater than about 10:1, andmore preferably greater than about 20:1.

Particle size may be measured in any appropriate manner in accordancewith conventional practices now or hereafter practiced. According to oneapproach, particle size may be determined by inspection of TEMinformation. Preferably, particle size measurements are made using alaser light diffraction particle size analyzer (such as a MalvernMastersizer X) using a dry powder feeder module. The measurement fromthis technique reports a particle size distribution curve, expressed interms of the volume of equivalent spheres. The numbers reported arediameters of spheres having a volume equivalent to the calculated volumeof the particles being measured. The D10, D50, and D90 values, forinstance, may be determined from the information embodied in thedistribution curve. D10 value refers to the diameter such that 10% ofthe area of the distribution curve is equal to or smaller than thatvalue. Values for D50 and D90 are determined in an analogous manner for50% and 90%, respectively. Throughout this specification, a reference toparticle size refers to the D50 (average particle size) unless otherwiseexpressly noted.

A variety of materials may be used as the host material, either singlyor in combination, in composite support media of the present invention.Examples include a wide range of materials such as particles, powder,pellet, granule, extrudate, fiber, shell, honeycomb, plate, combinationsof these, and the like. The particles can be regular in shape,irregular, dendritic, dendrite-free, or the like. Because the compositewill further incorporate nanoporous guest material in preferredembodiments, the host material need not be, but can be if desired,nanoporous. Particulate embodiments of host material include alumina,activated carbon, alumino-silicates, silicates, transition metal oxides,combinations of these and the like. Alumina and activated carbon arepreferred.

A preferred embodiment of host particles includes an activated carboncommercially available under the trade designation “Kuraray GG” fromKuraray Chemical Co., Ltd. (Japan). This material is nanoporous andmesoporous. The material contains potassium carbonate but is low inhalide content. The material is derived from coconuts.

Guest material of the present invention preferably comprises nanoporousparticles or powders that can coat or otherwise become associated withall or a portion of the surfaces of the host material via physical,chemical, electrostatic adhesion, or other means. Representativeexamples of guest particles include titania (preferably wherein at leasta portion of the titania is in the anatase crystalline form); zincoxide; ceria; iron oxide; alumina; tin oxide, silicon oxide;sol-gel-derived small particles; nanoporous, fine particle size zeolite;high surface area aerogel particles; combinations of these; and thelike. Titania is preferred.

In preferred embodiments, composite catalyst particles of the presentinvention are coated onto at least a portion of the surfaces offiltration media arrays such as those described in U.S. Pat. No.6,752,889 (the entirety of which is incorporated herein by reference) oras commercially available under the trade designation 3M High Air Flow(HAF) filters from 3M Company, St. Paul, Minn. These media generallyinclude a plurality of open pathways, or flow channels, extending fromone side of the media to the other. Even though the composite catalystparticles might only coat the surfaces of these channels, leaving largeopen volumes through the channels for air streams to pass, it has beenfound that substantially all CO in air streams passing through the medianonetheless is catalytically oxidized with virtually no pressure drop.Most preferably, the composite catalyst particles of this embodiment areformed from titania guest particles coated onto carbon host particles(such as the Kuraray GG activated carbon particles). The catalyticallyactive gold may be deposited onto the titania particles before thecomposite particles are assembled. Alternatively, the gold can bedeposited onto the assembled composite particles.

Heterogeneous catalyst systems of the present invention optionally mayincorporate one or more activating agents to enhance catalyticperformance of the system. As used herein, an activating agent generallyrefers to any ingredient that is generally not catalytic by itself yetcan enhance the performance of a catalyst when both the activatingagent(s) and the catalyst are incorporated into the system. In preferredembodiments, the activating agent(s) may be incorporated into thedesired support before, during or after gold deposition. Preferably,this incorporation occurs prior to gold deposition. In the case ofcomposite support materials comprising guest material provided on hostmaterial, the activating agent(s) may be incorporated into the hostmaterial and or the guest material.

One preferred class of activating agents of the present inventionincludes one or more metal salts. Water soluble salts such as alkalimetal salts and/or alkaline earth metal salts are inexpensive, readilyavailable, and easily incorporated into catalytic systems whenpracticing the present invention. Significantly, it has been discoveredthat these salts are potent activators for gold-based catalysis,especially when used to activate nanoporous carbon support media.Bifurcating PVD deposition of catalytically active gold from earlieractivation of the support media was a key to help enable this advance inthe use of carbon media supports with activating salts for gold-basedcatalysis.

Clearly, the use of such a metal salt enhances catalytic performance,but the exact mechanism by which performance is enhanced is not knownfor certain. Without wishing to be bound, it is believed that the metalcation reacts with the surface of the support in a manner that helps toimmobilize gold (e.g., by providing a multiphasic surface) and/or thatthe metal cation functions as an electron acceptor or participates insome fashion in the catalytic reaction sequence. The metal cation mayalso provide water adsorption sites to provide water to enhance thecatalytic reaction.

Examples of metal salts include salts of the alkali or alkaline earthmetals such as lithium, potassium, sodium, magnesium, calcium, and/orbarium. Other metals include Cs, Rb, and the like. Combinations of anyof these metal salts may be used. In some embodiments, the activatingagent comprises at least one alkali metal salt and at least one alkalineearth metal salt, wherein the weight ratio of the alkali metal salt tothe alkaline earth metal salt is in the range of about 1:19 to about19:1, preferably about 1:3 to about 3:1.

The metal salts can include any suitable counter anion(s). Examplesinclude nitrate, hydroxide, acetate, carbonate, combinations of these,and the like. Either carbonate or hydroxide is an especially preferredanion as it is safe and convenient to handle and forms very activesupports. If the nitrate anion is included, the substrate desirably iscalcined to a sufficiently high temperature to decompose the nitrateanion to activate the support. Carbonate is even more effective whenused in combination with an alkali metal or alkaline earth metal.Accordingly, preferred activating agents of the invention comprise acarbonate salt, and more preferably an alkali metal carbonate salt or analkaline earth metal carbonate salt.

Potassium carbonate is very effective, for example, especially when usedon activated carbon with a gold catalyst, but it is also effective insystems with other types of supports, e.g., alumina, as well. The factthat potassium carbonate activates a carbon-gold system is quitesurprising. Firstly, depositing gold onto K₂CO₃ in the absence of thecarbon or other nanoporous support provides a system with very low, ifany, catalytic activity. Further, depositing gold on activated carbon inthe absence of the K₂CO₃ also provides a system with very low, if any,catalytic activity. Yet, when the three ingredients are combined, a veryeffective catalyst system results. Indeed, discovering such a simple andeffective way to make activated carbon as a support for catalyticallyactive gold is a significant accomplishment. The benefits of thecarbonate is highlighted by data showing that potassium sulfate is apoor activating agent, although it is believed that its performancewould improve if a support impregnated with potassium sulfate were to bethermally treated prior to gold deposition.

Yet, potassium carbonate and many of the other salts mentioned hereinare very soluble in aqueous solution. Depositing the gold onto thesubstrate via PVD allows systems containing both gold and suchactivating materials to be easily made. Water soluble activators such asK₂CO₃ cannot be used with conventional aqueous impregnation orprecipitation methods. This is because they would dissolve in and bewashed from the support medium by the water solvents.

Another advantageous class of activating agents includes alkoxidematerials, especially those described above with respect to formingnanoporous surface features on less porous host particles. Preferredalkoxides include alkoxides of Ti and Al. Alkoxide materials areadvantageously used in combination with one or more of the water solublesalt materials described above. When the two kinds of materials are usedtogether, they can be impregnated onto the support at the same time orsequentially in any order, although it is preferred that the alkoxidematerial(s) be impregnated onto the support after the impregnation ofthe salt(s). In a representative process, the water soluble salt isimpregnated onto the support, and the support is then dried andoptionally calcined. Next, the alkoxide is impregnated onto the guestparticle, the product is hydrolyzed, dried, and optionally calcined.Thus, prepared, gold is then deposited onto the activated support.

Use of an alkoxide as an impregnant/activating agent appears to changethe crystalline structure of the support in our TEM studies.Specifically, the grain structure of the support proximal to the supportsurface appears to be much finer than the core region and much finerthan otherwise identical systems prepared without the alkoxide. Thestructure modification penetrates in most instances further into thesupport than the gold, e.g., 50 nm or more. In some instances, theboundary between the modified surface region and the unmodified coreregion is easily observed.

Not all alkoxides may work in all conditions. For example, Ti and Alalkoxides were found to enhance catalytic performance when incorporatedinto catalyst systems as shown in the examples. However, substituting aZr-based alkoxide into these formulations did not demonstrate anyenhancement in the ability of the system to oxidize CO.

In a similar fashion, some water soluble salt activating agents,particularly sulfates, oxalates, and phosphates, did not demonstrateactivating performance in some of our studies, although it is believedthat calcining the impregnated support could improve performance of atleast the sulfates and oxalates. While not wishing to be bound, it isbelieved that these kinds of anions, which tend to be coordinating,impact support surface charges in a manner that impairs the ability ofthe surface to immobilize gold. Yet, sulfate and oxalate anions arereadily decomposed at reasonable calcining temperatures, which explainswhy we believe that calcining would enhance the activatingcharacteristics of these materials.

Iron salts also are poor candidates for use as the only activating agentwhen PVD techniques are used to deposit gold. This is unexpected,inasmuch as iron salts are effective activators when gold is impregnatedonto particles via solution processing. This shows that ingredients thatreadily work in one context, e.g., solution processing, may not work thesame in another context, e.g., PVD processing.

Likewise, not all porous supports are readily activated under the sameconditions that work with other support media. For instance, certainzeolites, e.g., sodium Y zeolites form poor support media when processedin ways that are effective for alumina, carbon, silica, hopcalite, etc.Even when activated with a salt, low or no catalytic activity for COoxidation was observed when procedures that worked for alumina wereapplied to zeolite media. Zeolites are known to have more orderedstructures and to not possess the defects of other oxides. Silicalite,the aluminum-free form of ZSM-5-type zeolite, was found to work well inthe present invention. Thus, for certain zeolite materials to be used assupport media, they are preferably surface treated in some fashion toenhance the ability of the surface to immobilize gold.

The amount of activating agent used in the heterogeneous catalyst systemcan vary over a wide range and will depend upon a variety of factorsincluding the nature of the activating agent, the amount of gold to beincorporated into the system, the nature of the support, and the like.Generally, if too little activating agent is used, the potentialbenefits of using the activating agent may not be fully attained. On theother hand, beyond some point, using additional activating agent may notprovide significant additional benefit and may undermine catalyticperformance to some degree. Accordingly, as suggested guidelines,representative embodiments of the invention may include from 0.25 to 15,preferably 1 to 5 weight percent of activating agent based upon thetotal weight of activating agent and the support. When one or more watersoluble salts and one or more alkoxide materials are used incombination, the molar ratio of the salt(s) to alkoxide(s) ingredient(s)is in the range of 1:100 to 100:1, preferably 1:5 to 5:1.

The activating agent may be incorporated into the heterogeneous catalystsystem in a variety of different ways. In some instances, the support tobe used may inherently include a suitable activating agent. For example,activated carbon derived from coconut shell naturally includes potassiumcarbonate as a constituent. This kind of activated carbon provides anexcellent support for gold catalyst without requiring additionalactivating ingredients.

We have demonstrated the benefit of using activated carbon from coconuthusk as well as the benefit of using potassium carbonate as anactivating agent. Kuraray GC carbon and Kuraray GG carbon are bothderived from coconut shells. Kuraray GG carbon is the natural, resultantcarbon that includes potassium carbonate. Kuraray GC carbon is similarexcept that it has been acid washed and then extensively rinsed withwater to remove the potassium carbonate and other acid and water solubleconstituents. When gold is deposited onto these two carbons using PVD,the system derived from Kuraray GG carbon (includes the potassiumcarbonate) is a very good catalyst for CO oxidation, especially undermore humid conditions. On the other hand, the system derived fromKuraray GC carbon (essentially no potassium carbonate) has low activityfor CO oxidation in dry or humid environments. Further, if the KurarayGG carbon is washed to remove the potassium salt, catalyticfunctionality of the resultant system is significantly compromised.Catalytic activity can be recovered again if the washed Kuraray GGcarbon is impregnated with an activating agent prior to gold deposition,especially if the impregnated carbon is thermally treated (describedfurther below) prior to gold deposition.

TEM (transmission electron micrograph) examination of the gold depositedon Kuraray GG carbon particles by physical vapor deposition showed thepresence of nanoparticles and protodots (protodots being the very smallclusters of gold) both on the immediate surface of the support and inpores immediately adjacent to the support surface. As could be seen inthe transmission electron micrograph, the gold was present in bothnanoparticle and in very small cluster forms. The gold particles formedpreferentially in small grooves and fissure-like pores in the carbon asevidenced by the orientation of the gold particles in linear,necklace-like patterns on the surface of the carbon. The dark fieldimage of the same region showed the gold-enriched striations clearly.The uniformity of the gold deposition could be clearly seen on the TEMimages. The gold clusters that were observed by TEM were as small as 1nm or less and as large as about 5 nm. The gold rich grooves orstriations were as wide as about 7 nm and as long as about 50 to 100 nm.There were also gold-rich domains comprising exceedingly fine goldarrays that appear as veil-like bright regions in the dark field image.It is unknown why these regions, although quite crystalline incharacter, did not coalesce into single crystals of gold.

Although not wishing to be bound by theory, a possible explanation forthe performance of potassium carbonate is that potassium carbonateprovides sites where water can adsorb. Indeed, in certain cases we havefound that the gold catalysts are more active in the presence ofmoisture.

Unlike Kuraray GG carbon, many other desirable supports do not naturallyinclude an activating agent. Consequently, in some instances, it may bedesirable to incorporate an activating agent comprising one or moreconstituents into the desired support. Such incorporation can occur inany desired manner. Incipient wetness impregnation is one suitabletechnique, and examples of using solution impregnation are described inthe examples below. Briefly, incipient wetness impregnation involvesslowly adding a solution comprising the desired activating agent to drysupport particles with mixing. The solution generally is added untilsaturation, and adding an excess of solution is desirably avoided. Suchsolutions typically are aqueous and the concentration of each species ofactivating agent in the solution generally is in the range of from about0.2 M to about 1.0 M. If more than one species of activating agent is tobe added, these may be added together, separately, or in overlappingfashion. After impregnation, the particles are dried and optionallycalcined (thermal treatment).

In any embodiments of the invention, the catalyst system may furtherincorporate one or more agents to enhance the filtering capabilities ofthe system. In many embodiments, such agents are in the form of one ormore impregnants that may be incorporated into the nanoporous supportmedium. In those embodiments wherein the nanoporous support medium has aguest/host composite structure, such impregnants may be incorporated inthe guest and/or host material. Most preferably, particularly when thehost comprises a carbonaceous material such as activated carbonparticles, the impregnants are incorporated at least into thecarbonaceous material.

Examples of impregnants include one or more metals, metal alloys,intermetallic compositions, and/or compounds containing one or more ofCu, Zn, Mo, Cr, Ag, Ni, V, W, Y, Co, combinations thereof, and the like.However, because the hexavalent form of Cr has been identified as apotential carcinogen, the catalyst system of the present inventionpreferably includes no detectable amounts of Cr (VI), and morepreferably no detectable Cr of any valence state due to the risk thatother forms of Cr, e.g., Cr(IV) could be oxidized to Cr(VI). The metalstypically are impregnated as salts and can be converted to other forms,e.g., oxides perhaps, during some modes of impregnation.

The selection of which one or more transition metal compounds toincorporate into the catalyst system depends upon the desired range offiltering capabilities inasmuch as each of the various transition metalstend to provide protection against particular air contaminants. Forexample, Cr, Mo, V, and Y or W independently help to filter gases suchas cyanogen chloride and hydrogen cyanide from air streams when used incombination with a Cu impregnant. Representative catalyst systemparticles may include 0.1 to 10 weight percent of one or moreimpregnants including Mo, V, W, and/or Cr. Due to the potential toxicityof Cr, the use of Mo, V, and/or W materials are preferred. Throughoutthis specification and accompanying claims, weight percent with respectto impregnants is based upon the total weight of the impregnatedparticles unless otherwise noted.

Cu tends to help filter many gases such as HCN, H₂S, acid gases, and thelike from air streams. Representative filter media particles may include0.1 to 15 weight percent of one or more impregnants including Cu.

Zn in various forms tends to help filter HCN, cyanogen chloride,cyanogen, and NH₃ from air streams. Representative filter mediaparticles of the present invention may include 1 to 20 weight percent ofone or more impregnants including Zn.

Ag tends to help filter arsenical gases from an air stream. Ag functionscatalytically and generally is not consumed during filtering operations.Accordingly, filter media particles may include relatively smallcatalytic amounts, e.g., about 0.01 to 1, preferably 0.1 weight percent,of one or more Ag-containing impregnants.

Ni and Co each independently helps to filter HCN from air streams.Representative filter media particles may include 0.1 to 15 weightpercent of one or more Ni containing impregnants and/or Co containingimpregnants.

In addition to one or more impregnants that contain transition metals,the first plurality of substrate particles may optionally include one ormore other kinds of impregnants. For example, ammonia or ammonium saltsin the impregnating solution not only help to improve the solubility oftransition metal compounds during the manufacture of the particles, butremaining adsorbed quantities also help to remove acid gases from airstreams. Sulfate salts are believed to help to control the pH duringusage of filter media. Ammonium sulfate, for instance, when impregnatedon a substrate such as carbon and dried at 145° C. forms an acidsulfate. Acid sulfate is sufficiently acidic to react with ammonia tofacilitate removal of ammonia from a flow of air or other gas. Throughimpregnation and drying, strongly acidic ammonium salts impregnate thecarbon during the drying process without damaging the basicoxide/hydroxide impregnant being formed. This results in enhancedammonia service life of a cartridge containing the resultant impregnatedcarbon. Representative filter media particles may include 0.1 to 10,preferably 2.5 to 4.5 weight percent of sulfate.

Moisture beneficially helps to remove acid gases from air streams.Optionally, therefore, the first plurality of filter media particles mayinclude up to about 15 weight percent, preferably about 6 to 12 weightpercent of water.

Impregnants may be incorporated into the catalyst system in accordancewith conventional practices. Such impregnants are typically provided assalts, oxides, carbonates, or the like and are impregnated via solutionprocessing, sublimation processing, fluidized bed processing, and thelike. Preferably, such impregnation occurs prior to gold deposition.Representative techniques for such processing have been widely describedin the literature, including the patent and literature documents citedin the Background section herein.

Deposition of gold preferably occurs via PVD after impregnation, coatingof host material or structures with at least one guest material (withrespect to host/guest composite structures when used), drying, andoptional calcining Bifurcation of impregnation and gold deposition is adistinct advantage for many reasons. First, if the gold were to be addedto the particles via solution impregnation, the kinds of activatingagents that could be used would be limited. For instance, HAuCl₄, a goldspecies commonly used in solution methods because of its relatively lowcost, is very acidic making it incompatible with basic activating agentssuch as the preferred alkali and alkaline earth metal salts. In caseswhere basic gold species are used, the aqueous impregnation would tendto wash away some of the desired activating ions. Thus, subsequentdeposition of gold via PVD (a non-solution process) separate fromimpregnation with activating agents is a significant process featurethat allows gold to be substantially more easily used in combinationwith these extremely effective activating agents. As an additionaladvantage, this method allows gold to be deposited onto the support withthe activating agent already in place. We think this is one reason whygold deposited in accordance with our invention is so active asdeposited without requiring a subsequent thermal treatment.

Thermal treatment (calcining) of the activated support prior to golddeposition, however, can be very beneficial. In some instances, anactivating agent may not function to the desired degree until aftercalcining For example, calcining tends to yield demonstrableimprovements when the activating agent includes a nitrate salt. In otherinstances, the performance of an effective activating agent would befurther enhanced. For example, the performance of generally effectivecarbonate salts can be enhanced to a degree via calcining Yet, saltssuch as potassium carbonate tend to already be in active form whenimpregnated, and the resultant activated supports are beneficiallydried, e.g., at a temperature up to about 200° C. without really needinga calcining treatment.

In general, thermal treatment involves heating the impregnated supportat a temperature in the range of 125° C. to about 1000° C. for a timeperiod in the range of 1 second to 40 hours, preferably 1 minute to 6hours, in any suitable atmosphere, such as air; an inert atmosphere suchas nitrogen; carbon dioxide; argon; or a reducing atmosphere such ashydrogen; and the like. The particular thermal conditions to be usedwill depend upon factors including the nature of the support and thenature of the impregnants(s). Generally, thermal treatment should occurbelow a temperature at which the constituents of the impregnated supportwould be decomposed, degraded, or otherwise unduly thermally damaged.Many calcining treatments of impregnated supports are described in theexamples below.

Although an activating agent may be supplied as a salt or the like, theresultant form of the salt or its constituent ions after incorporationinto the heterogeneous catalyst system is not known with certainty.Analysis by x-ray diffraction shows no distinct oxide or carbonate phaseof metal, although some carbonate per se is shown. It is believed,therefore, that the metal ions have reacted with and modified thesupport surface.

There is a wide range of applications for catalysts of the presentinvention. We believe that these catalysts will find application in theareas of treatment of automobile exhaust, as hydrogenation catalysts, ascatalysts for the oxidation of hydrocarbons, and as catalysts for theremoval of the oxides of nitrogen, and in sensors for detection andmeasurement of gases and vapors, and CO removal from inhabited areas.Respiratory protection devices such as smoke masks or escape hoods couldusefully employ catalysts of the invention for the removal of hazardousCO or other gases from breathing air.

The catalysts of the present invention have been shown to be suitablefor use in the demanding application of removal of CO from gas streamsin automotive cabin air purification. In this application, large volumesof gas and high flow rates can be encountered. Thus, the residence timeof the cabin air on the catalyst is short, being less than 0.05 secondsand even less than 0.03 seconds. Typically, the CO levels encounteredare low, usually less than 200 ppm. The catalysts of the presentinvention have been shown to perform very well in these conditions andcan be used in a variety of configurations in cabin air purification fora wide range of vehicles and vessels that carry passengers.

The present invention will now be further described in the followingillustrative examples.

Test Procedure 1: CO Challenge Testing of 50 to 200 mL of GranularCatalysts in a Fixture

FIG. 4 a shows testing system 50 used to subject catalyst samples to COchallenges in order to assess their performance as CO oxidationcatalysts. High-pressure compressed air from supply line 52 is reducedin pressure, regulated, and filtered by regulator 54 (3M Model W-2806Air Filtration and Regulation Panel, 3M, St. Paul, Minn.) to removeparticulates and oils. Valve 56 (Hoke Inc., Spartanburg, S.C.) is usedto set the desired main airflow rate as measured by a flow meter 58(Gilmont®, Barnant Co, Barrington, Ill.) with a range of 0 to 90 LPM.The flow meter 58 was calibrated using a dry gas test meter (AmericanMeter, model DTM-325; not shown). The main airflow passes through theheadspace 60 above a heated distilled water bath 62 of vessel 64 andthen passes via lines 57 and 77 into a 500 ml mixing flask 66. Relativehumidity in the mixing flask is monitored using a RH sensor 68 (Type850-252, General Eastern, Wilmington, Mass.). The RH sensor 68 providesan electrical signal to a humidity controller 70 (an Omega EngineeringPID controller series CN1200 from Omega Engineering Inc., Stamford,Conn.) that delivers power via lines 71 to a submerged heater 72 tomaintain the RH at the set point. Unless otherwise indicated, therelative humidity is controlled at >90%.

A cylinder 74 of carbon monoxide (Praxair, Inc., Danbury, Conn. 98.5%)equipped with a regulator 76 suitable for CO service provides aregulated flow of CO gas via line 73. A Gilibrator® bubble flow meter 75(Sensidyne, Inc., Clearwater, Fla.) measures volumetric CO flow rate inthe range 20 mL/min to 6 L/min. A stainless steel, fine metering valve78 (Swagelok Co, part SS-SS2, Solon, Ohio) is used to set the desired COflow rate. The metered CO is combined with the humidified air in themixing flask 66.

This system can deliver mixtures of CO and air at concentrations fromabout 1000 ppm CO to about 20,000 ppm CO at flow rates from about 15L/min to about 80 L/min at RH values from about 5% to approximately 95%.More dilute mixtures for detector calibration were generated byreplacing the cylinder of carbon monoxide 74 with a cylinder of acertified mixture of CO in air or nitrogen (typically from 500 to 5000ppm CO; Quality Standards, Pasadena, Tex.).

The combined stream then flows via line 85 into an inverted, 12-quart,stainless steel beaker 80 with a 29/42 outer joint welded into the topclosably engaging a support platform 83 defining test chamber 81. Insidethe beaker 80 is a test fixture 82. The beaker 80 is sealed to supportplatform 83 using a foam gasket (not shown). Two clamps (not shown)ensure a tight seal to the support platform 83. The beaker 80 can beremoved to allow catalyst test fixtures to be placed inside for testingand taken out after testing is complete. The support platform 83 isequipped with an inner 29/42 tapered fitting (not shown) onto which thefixture 82 containing the catalyst to be tested is mounted. A drawing ofthe fixture 82 is shown in FIG. 2 of ASTM Standard Guide for Gas-PhaseAdsorption Testing of Activated Carbon D5160-95.

The CO concentration from the test stream emerging from the outlet ofthe test chamber is measured by CO detector system 84. The results areprocessed via computer 86. Two embodiments of CO detector system 84 arepreferred. In one embodiment, CO detector system 84 includes an SRI8610C gas chromatograph (SRI Instruments, Torrance, Calif.) equippedwith a gas sampling valve and a detector that responds to CO. Adiaphragm pump (KNF Neuberger, Inc., Trenton N.J. UNMP830 KNI)continuously draws approximately 50 mL/min of sample from the testoutlet through the gas sampling valve of the GC. Periodically the valveinjects a sample onto a 3 ft 13× molecular sieve column. CO is separatedfrom air and its concentration measured by either a helium ionizationdetector (minimum detectable CO concentration about 10 ppm) or amethanizer/FID detector (minimum detectable CO concentration less than 1ppm). The GC is calibrated using CO in air mixtures generated using thetest system described above. Results of this calibration agree to within3% of those from certified standard CO in air or nitrogen mixtures inthe range from 500 to 5000 ppm CO (Quality Standards, Pasadena, Tex.).Each CO analysis takes about 3 minutes. After completion of theanalysis, another sample is injected onto the column and the analysisrepeated.

In another embodiment, CO detection system 84 uses a Brüel & KjærMultigas Monitor Type 1302 (Brüel & Kjæer, Nærum, Denmark; not shown)equipped with optical filter #984 to detect CO. The Multigas Monitor iscalibrated using the test system described above. The temperature of theair stream is monitored downstream of the test fixture using a K-typethermocouple (not shown) and digital readout (not shown) (Fluke 51 K/JThermometer, Fluke Corporation, Everett, Wash.).

Catalyst samples are sieved to remove fines prior to testing. Unlessotherwise specified, samples were sieved to remove particles finer than25 mesh using ASTM E 11 U.S. Standard Sieves. A specified catalystvolume, typically 100 ml, is loaded into the 3.5 inch inner diameteraluminum test fixture 82. The fixture is loaded with catalyst using asnowstorm filling technique in which the catalyst falls into the testfixture 82 through a loading column containing screens to evenlydistribute the catalyst across the bed. Typical bed depth isapproximately 1.6 cm (0.6 in). To begin the test, test fixture 82containing the catalyst is placed on the 29/42 fitting on the supportplatform 83. The beaker 80 is replaced and sealed to the supportplatform 83. Outlet CO concentration measurements begin when the CO/airmixture is introduced into the test chamber 81. Measurements continuefor a specified time period, typically 30 minutes.

Test Procedure 2: CO Challenge Testing of 2.5 to 10 ml of GranularCatalysts (Tube Test)

FIG. 4 b shows a test system 250 used to quickly screen small quantitiesof new catalyst formulations for activity. While system 50 (See FIG. 4a) used in test procedure 1 requires from about 50 to 200 mL of sample,the system 250 of this procedure allows testing to be done with samplevolumes ranging from about 2.5 to about 10 mL. The system 250 used totest CO/air mixtures in this procedure is similar to system 50, but witha few differences. Features of system 250 that are identical to similarfeatures of FIG. 4 b are identified by the same reference numeralsincremented by 200. A polycarbonate box 287 with 29/42 connections (notshown) at the top and bottom replaces the stainless steel beaker 80 andsupport platform 83 of system 50. A CO/air mixture of the desiredconcentration flows into this box 287 via line 285 typically at 64 L/minand >90% RH. A portion of this flow (from about 1 L/min to about 10L/min) is pulled through a tube 289 containing the catalyst sample 290while the excess is vented outside the box 287 via vent 291.

The catalyst sample 290 of known volume is loaded into tube 289 (a ⅝inch ID (¾ inch OD) copper tube about 3.5 inches in length sealed at oneend by a cotton plug (not shown)). The volume of the sample isdetermined by loading it into a graduated cylinder using the methoddescribed in ASTM D2854-96 Standard Method for Apparent Density ofActivated Carbon. This measured volume is then loaded into the coppertube using the same method.

The tube 289 containing the catalyst sample 290 is introduced up throughthe 29/42 inner fitting at the bottom of the polycarbonate box 287 sothat the open end extends into the box. The other end of the tube isequipped with a ¾ inch Swagelok® nut and ferrule (not shown) for easyconnection and disconnection to/from the test system 250. The nutengages a female fitting (not shown) in a ½ inch OD tube 295 connectedvia a branch 296 to a vacuum source (not shown) through a rotameter 293and needle valve 294. The tube 295 also connects to the inlet of thediaphragm pump (not shown) via branch 297 which draws sample to thesampling valve of a gas chromatography instrument and CO detector usedas CO detection system 284 (just as in test procedure 1). The small flowto the gas chromatography instrument (approximately 50 mL/min) isusually negligible in comparison to the total flow through the catalystbed. The rotameter 293 is calibrated by placing a Gilibrator soap bubbleflow meter (not shown) at the entrance to the copper tube containing thecatalyst.

To start the test, a steady 64 L/min flow of a CO/air mixture of thedesired concentration and RH is introduced into the polycarbonate box287. The needle valve 294 is then adjusted to give the desired flowthrough the catalyst sample 290. The CO concentration in the air exitingthe catalyst sample 290 is analyzed by the CO detection system 284 as intest procedure 1.

Test Procedure 3: CO Challenge Testing of Powder Samples (Less than 1ml)

FIG. 5 shows a system 100 used for analysis of the catalyticcharacteristics of catalysts in the form of fine powders. System 100includes a high pressure CO in air mixture (1.9% v/v) stored in tank 102and is also coupled to a source 104 of building compressed air via line106. Pressure regulator and shut off valve 101 and fine needle valve 103help control the flow of CO in air through line 105. The flow of CO inair is measured by rotameter 107 (Alphagaz (Air Liquide) 3502 flow tube,Morrisville, Pa.).

The flow of building compressed air through line 106 is reduced inpressure, regulated, and filtered by regulator 110 (3M Model W-2806 AirFiltration and Regulation Panel, 3M Company, St. Paul, Minn.). Fineneedle valve 112 is used to set the desired airflow rate through line106.

The flows of CO in air and the building compressed air are combined atjuncture 108 to provide a CO in air mixture of the desired concentrationand flow rate. Rotameter 114 (Aalborg Instruments 112-02 flowtube,Orangeburg, N.Y.) on line 116 measures the total flow of the combinedstreams. The rotameters are calibrated using a Gilibrator® bubble flowmeter (not shown) placed at the catalyst bed. The two rotameters 107 and114 are chosen to obtain concentrations from about 1000 to 20,000 ppm COat flow rates from about 100 mL/min to about 500 mL/min under labambient conditions.

The diluted CO in air mixture in line 116 is then humidified to thedesired RH by passing the air mixture through the inner tube of a tubein shell Nafion® humidifier 118 as shown (Perma Pure MH 070-12P; TomsRiver, N.J.). Water is introduced to the humidifier 118 through line 120and exits via line 122. This humidifier humidifies a CO/air streamto >90% RH at flow rates up to at least 400 mL/min. This was verifiedwith a General Eastern Hygro-M1 optical mirror dew point hygrometer (GEGeneral Eastern Instruments, Wilmington Mass.).

The catalyst sample (usually about 1-2 cm in depth) is snowflaked into asection of thick walled 4 mm ID polyethylene tubing 124 about 8 cm inlength to form a catalyst bed 125. A cotton plug (not shown) seals oneend of tubing 124. The CO in air mixture passes through the catalyst bedand then through a particulate cartridge filter 126 (Balston DFU samplefilter grade BQ, Parker Hannifin Corporation, Tewksbury, Mass.) into aSRI 8610C gas chromatograph (SRI Instruments, Torrance, Calif.) equippedwith a gas sampling valve and a detector that responds to CO. Theparticulate filter 126 protects the gas sampling valve from damage byparticulates escaping the catalyst bed.

The gas sampling valve periodically injects the exit stream from thecatalyst bed onto a 3 ft molecular sieve 13× column. This separates COfrom air. CO concentration is determined by either a helium ionizationdetector (CO detection limit≈10 ppm) or a methanizer/hydrogen flameionization detector (CO detection limit less than 1 ppm). The COconcentration is measured approximately every four minutes during thetest, displayed, and recorded in a data file. Calibration is by themethod described in test procedure 1.

Gold Application Method: Process for Deposition of Gold Nanoparticlesonto Substrate particles:

The apparatus described in the detailed description and shown in FIGS. 2and 3 is used as follows to prepare catalytic materials according to thefollowing procedure, unless expressly noted otherwise. 300 cc ofsubstrate particles are first heated to about 200° C. in air overnight(120° C. in the case of carbon substrates) to remove residual water.They are then placed into the particle agitator apparatus 10 while hot,and the chamber 14 is then evacuated. Once the chamber pressure is inthe 10⁻⁵ ton range, the argon sputtering gas is admitted to the chamber14 at a pressure of about 10 millitorr. The gold deposition process isthen started by applying a pre-set power to the cathode. The particleagitator shaft 40 is rotated at about 4 rpm during the gold depositionprocess. The power is stopped after the pre-set time. The chamber 14 isbackfilled with air and the gold coated particles are removed from theapparatus 10. The gold sputter target 32 is weighed before and aftercoating to determine the amount of gold deposited. In general, about 20%of the weight loss of the target represents gold deposited on thesample.

During the deposition process the gap between the blades 42 and thechamber wall was set to a pre-set value, e.g., 1.7 mm to 2.7 mm or evenmore if comminution is desirably avoided.

Example 1 Sputtered Au on TiO₂/Kuraray GG Carbon: Effect of Step Changein CO Inlet Concentration

This sample was prepared by sputter coating gold onto 300 mL of KurarayGG 12×20 mesh, activated carbon that had been coated with 10% IshiharaST-31 titania dispersed in DI water.

22.1 grams of ST-31 titania (Ishihara Sangyo Kaisha, LTD, Tokyo, Japan)was dispersed into 160 grams of deionized water utilizing an IKA UltraTurrax T18 homogenizer (IKA Works, Inc., Wilmington, Del.). The slurrywas spritzed (pumping the titania dispersion with a peristaltic pump(Cole Palmer Instruments Co, Chicago, Ill., model wz1r057) at 150 gr/minthrough a Qorpak finger-actuated trigger-sprayer (Qorpak, Bridgeville,Pa.) as a fine mist) onto 200 grams of Kuraray GG 12×20 mesh activatedcarbon that was mixed in a 1 gallon steel reactor rotating at 18 rpm at20 degree angle. After the granules were coated, a heat gun was used toremove enough water to allow the granules to flow freely in the rotatingreactor. The coated granules were dried at 120 C. in an oven forapproximately 2 hours giving a uniform white coating. The sample wasfurther dried at 150° C. for 24 hours. 129.54 grams of the sample wascoated with 0.88 grams gold (weight loss from the target) deposited viaPVD. The coater used an agitator with a height of 2.7 cm, blade gap of1.7 mm, and the holed blade was rotated at 4 rpm. The backgroundpressure was 8.7×10⁻⁵ ton. The sputter power was 0.03 kw for 1 hour.

100 mL of the sample (43.1 g) was tested (test procedure 1) against a COchallenge at 30 L/min in a 3.5 inch diameter aluminum test fixture. Testrelative humidity (RH) was 93%. A thermocouple at the filter exitmeasured air temperature. The GC used in this test is equipped with amethanizer/FID detector that is capable of detecting CO at levels below1 ppm. The initial CO challenge concentration at the beginning of thetest was 2500 ppm. No CO was detected at the filter outlet during thecourse of this challenge. The temperature of the air at the filteroutlet (open circles on the graph) increased steadily and reached about41° C. after about 40 minutes. At about 42 minutes into the test, theinlet CO concentration was increased suddenly to 10,000 ppm. Still, noCO was observed at the filter outlet. Air temperature at the outletbegan to rise sharply and reached about 96° C. at the end of the test.The fast response to a sudden change in inlet CO concentration observedwith this catalyst in this test is highly desirable in a respiratorfilter against CO. The test results are shown in FIG. 10.

The data depicted by the open circles shows the temperature at theoutlet during the course of the test. The data depicted by the blacktriangles shows the measured CO content at the outlet. The response ofthe filtering system to the stepwise increase in CO in the challengestream was so rapid that no discernible spike in CO at the outlet wasdetected.

Example 2 Sputtered Au on TiO₂/Kuraray GG Carbon: Lesser Amount ofCatalyst Material

The test of Example 1 was repeated, except only 50 mL of the samecatalyst was used. Further, the CO challenge was held at 2500 ppm (nostepwise increase to 10,000 ppm was used in this test). The test resultsare shown in FIG. 11. The data depicted by the open circles show thetemperature data at the outlet, and the data depicted by the blacktriangles show the detected CO data at the outlet. No CO was detected atthe filter outlet during the entire 120 minutes of testing. Thetemperature of the air at the filter outlet (open circles on the graph)increased slightly and then remained steady at about 42.7° C. Thisexample illustrates the extremely high activity for CO oxidation of thecatalysts of the present invention.

Example 3 Sputtered Au on TiO₂/Kuraray GG Carbon: Effect of Step Changein CO Inlet Concentration at Higher Flow Rate

This sample was prepared by sputter coating gold onto 300 mL of KurarayGG 12×20 activated carbon that had been coated with 10% Hombikat UV100titania dispersed in DI water. 22.1 grams of Hombikat UV 100 (SachtlebenChemie Gmbh, Duisburg, Germany) was dispersed into 150 grams ofdeionized water utilizing an IKA Ultra Turrax T18 homogenizer (IKAWorks, Inc., Wilmington, Del.). The slurry was spritzed (pumping thetitania dispersion with a peristaltic pump (Cole Palmer Instruments Co,Chicago, Ill., model wz1r057) at 150 gr/min through a Qorpakfinger-actuated trigger-sprayer (Qorpak, Bridgeville, Pa.) as a finemist) onto 200 grams of Kuraray GG 12×20 mesh activated carbon that wasmixed in a rotating 1 gallon steel reactor rotating at 18 rpm at 20degree angle. After the granules were coated, a heat gun was used toremove enough water to allow the granules to flow freely in the rotatingreactor. The coated granules were dried at 120° C. in an oven forapproximately 2 hours giving a uniform white coating. The sample wasfurther dried at 150° C. for 24 hours. 126 grams of the sample wascoated with 1.34 grams gold (weight loss from the target). The coaterused an agitator with a height of 2.7 cm, blade gap of 2.7 mm, and theholed blade was rotated at 4 rpm. The background pressure was 8.50×10⁻⁵torr. The sputter power was 0.04 kW for 1 hour.

50 mL of the sample was tested (test method 1) as in Example 1, but at64 L/min flow instead of the 30 L/min. As further differences, the COchallenge concentration at the beginning of the test was 1200 ppm, andthe inlet CO concentration was increased suddenly to 10,000 ppm at about60 minutes. AP of the sample in the fixture was 18.5 mm H₂O at 85 L/min.Test RH was >90%.

The results are shown in the graph of FIG. 12. The data depicted by theopen circles show the temperature data at the outlet, and the datadepicted by the black triangles show the detected CO data at the outlet.CO concentration at the outlet of the filter quickly reached a plateauof about 10 to 11 ppm. Almost no temperature spike from the interactionof water vapor in the wet challenge gas with the dry catalyst wasobserved. The temperature of the air at the filter outlet (open circles)remained steady at about 36° C. After the CO challenge was stepwiseincreased to 10,000 ppm, the CO concentration at the outlet moderatelyspiked upward but quickly dropped back to 12-15 ppm. Air temperature atthe outlet began to rise sharply and reached about 109° C. After about90 minutes, the CO inlet concentration was reduced back to 1200 ppm. Thecatalyst continued to function even as the temperature dropped to lessthan 50° C. The catalyst in this example increased in weight by 0.8 gduring the test.

The outlet CO concentration during this test also was monitored with aB&K gas analyzer. This device can analyze the CO content of a sampleabout every minute while the GC takes about 3 minutes to complete ananalysis. The maximum CO concentration measured by the B&K was 111 ppm.

The fast response to a sudden change in inlet CO concentration observedwith this catalyst in this test is highly desirable in a respiratorfilter against CO. The response of this sample to the stepwise increasewas much faster and much more moderate than the response of the samplein Comparative Example A (below) in which an alumina host was used.

Example 4 12.5 wt Percent Hombikat on GG Carbon Coated in a DeionizedWater Slurry

28.5 grams of Hombikat UV 100 (Sachtleben, Germany) was dispersed into160 grams of deionized water utilizing an IKA Ultra Turrax T18homogenizer (IKA Works, Inc., Wilmington, Del.). The slurry was spritzed(pumping the titania dispersion with a peristaltic pump (Cole PalmerInstruments Co, Chicago, Ill., model wz1r057) at 150 gr/min through aQorpak finger-actuated trigger-sprayer (Qorpak, Bridgeville, Pa.) as afine mist onto 200 grams of Kuraray GG 12×20 mesh activated carbon thatwas mixed in a 1 gallon steel reactor rotating at 18 rpm at 20 degreeangle. After the granules were coated, a heat gun was used to removeenough water to allow the granules to flow freely in the rotatingreactor. The coated granules were dried at 120° C. in an oven forapproximately 2 hours giving a uniform white coating. The sample wasfurther dried at 150° C. for 24 hours. 126 grams of the sample wascoated with 6.98 grams gold (weight loss from the target) using PVDtechniques. The coater used an agitator with a height of 2.7 cm and theholed blade was rotated at 4 rpm. The background pressure was 7.90×10⁻⁵torr. The sputter power was 0.12 kW for 2 hour.

Example 5 10 wt Percent ST-31 on GG Carbon Coated in a Deionized WaterSlurry

22.1 grams of ST-31 (Ishihara, Japan) was dispersed into 160 grams ofdeionized water utilizing an IKA Ultra Turrax T18 homogenizer (IKAWorks, Inc., Wilmington, Del.). The slurry was spritzed (pumping thetitania dispersion with a peristaltic pump (Cole Palmer Instruments Co,Chicago, Ill., model wz1r057) at 150 gr/min through a Qorpakfinger-actuated trigger-sprayer (Qorpak, Bridgeville, Pa.) as a finemist onto 200 grams of Kuraray GG 12×20 mesh activated carbon that wasmixed in a 1 gallon steel reactor rotating at 18 rpm at 20 degree angle.After the granules were coated, a heat gun was used to remove enoughwater to allow the granules to flow freely in the rotating reactor. Thecoated granules were dried at 120° C. in an oven for approximately 2hours giving a uniform white coating. The sample was further dried at150° C. for 24 hours. 133 grams of the sample was coated with 3.56 gramsgold (weight loss from the target) using PVD techniques. The coater usedan agitator with a height of 2.7 cm and the holed blade was rotated at 4rpm. The background pressure was 1.020×10⁻⁵ torr. The sputter power was0.12 kW for 1 hour.

Example 6 12.5 wt Percent ST-31 on GG Carbon Coated in a 0.5M KOH Slurry

28.5 grams of ST-31 (Ishihara, Japan) was dispersed into 160 grams of0.5 M KOH utilizing an IKA Ultra Turrax T18 homogenizer (IKA Works,Inc., Wilmington, Del.). The slurry was spritzed (pumping the titaniadispersion with a peristaltic pump (Cole Palmer Instruments Co, Chicago,Ill., model wz1r057) at 150 gr/min through a Qorpak finger-actuatedtrigger-sprayer (Qorpak, Bridgeville, Pa.) as a fine mist onto 200 gramsof Kuraray GG 12×20 mesh activated carbon that was mixed in a 1 gallonsteel reactor rotating at 18 rpm at 20 degree angle. After the granuleswere coated, a heat gun was used to remove enough water to allow thegranules to flow freely in the rotating reactor. The coated granuleswere dried at 120° C. in an oven for approximately 2 hours giving auniform white coating. The sample was further dried at 150° C. for 24hours. 143 grams of the sample was coated with 6.56 grams gold (weightloss from the target) using PVD techniques. The coater used an agitatorwith a height of 2.7 cm and the holed blade was rotated at 4 rpm. Thebackground pressure was 9.0×10⁻⁵ torr. The sputter power was 0.24 kW for1 hour.

Example 7 10 wt Percent ST-31 on GG Carbon Coated in a 0.5M KOH Slurry

22.1 grams of ST-31 (Ishihara, Japan) was dispersed into 160 grams of0.5 M KOH utilizing an IKA Ultra Turrax T18 homogenize (IKA Works, Inc.,Wilmington, Del.). The slurry was spritzed (pumping the titaniadispersion with a peristaltic pump (Cole Palmer Instruments Co, Chicago,Ill., model wz1r057) at 150 gr/min through a Qorpak finger-actuatedtrigger-sprayer (Qorpak, Bridgeville, Pa.) as a fine mist onto 200 gramsof Kuraray GG 12×20 mesh activated carbon that was mixed in a rotating 1gallon steel reactor rotating at 18 rpm at 20 degree angle. After thegranules were coated, a heat gun was used to remove enough water toallow the granules to flow freely in the rotating reactor. The coatedgranules were dried at 120° C. in an oven for approximately 2 hoursgiving a uniform white coating. The sample was further dried at 150° C.for 24 hours. 137 grams of the sample was coated with 6.34 grams gold(weight loss from the target) using PVD techniques. The coater used anagitator with a height of 2.7 cm and the holed blade was rotated at 4rpm. The background pressure was 1.06×10⁻⁵ torr. The sputter power was0.24 kw for 1 hour.

Example 8 CO Catalytic Performance for Samples 4-7

The following table shows the performance of Samples 4 through 7 withrespect to CO oxidation. The performance of these samples is alsodepicted in FIG. 9.

Sample CO % CO % CO % CO % CO % CO % CO % Number Sample Description T0.5T5 T10 T15 T20 T25 T30 4 12.5% Hombikat DI 99.9 99.8 99.8 99.7 99.6 99.599.5 dispersed sprayed on GG Carbon, 6.98 g Gold 4 12.5% Hombikat DI 9699 99.5 99.9 99.9 99.9 99.9 dispersed sprayed on GG Carbon, 6.98 g Gold,Aged 7 days 70 C. 5 10% ST-31 DI dispersed 91 99 99 99 99 99 99 sprayedon GG carbon, 3.56 g Gold 6 12.5% ST-31 0.5M KOH 86.3 99.6 99.5 99.499.4 99.3 99.3 dispersed sprayed on GG Carbon, 6.56 g Gold 7 10% ST-310.5M KOH 92.8 99.4 99.3 99.2 99.1 99 98.9 dispersed sprayed on GGCarbon, 6.34 g Gold

Comparative Example A Sputtered Au on TiO₂/Washed Alcoa 450 Alumina:Effect of Step Change in CO Inlet Concentration

This sample was prepared by sputter coating gold onto 300 mL of washedAlcoa 450 alumina beads that had been coated with 10% Hombikat UV100titania dispersed in 0.5 M K₂CO₃ solution.

The titania coated Alcoa 450 alumina beads were prepared in thefollowing manner. A colorant dispersion comprising γ-Fe₂O₃ in water wasprepared by washing 30.07 g of γ-Fe₂O₃ (Sigma Aldrich Fine Chemicals,Milwaukee, Wis.) with 500 ml of deionized water by centrifugation. Thismaterial was then washed with 500 ml of a potassium carbonate solutionprepared by dissolving 3.0 g of potassium carbonate in 500 ml ofdeionized water. The material was final washed with 500 ml of deionizedwater and was separated by centrifugation. The recovered solid wasredispersed in deionized water to a concentration of 1.64% iron oxide byweight. This dispersion was used to provide color to the titaniadispersion so as to be able to monitor the uniformity of the titaniacoated onto the Alcoa 450 alumina beads.

To prepare the Alcoa 450 alumina beads for use, about 1 liter of beadswere immersed in 2 liters of deionized water. These were poured into alarge buchner funnel attached to a filter flask. The wash water wasallowed to drain from the funnel into the flask by means of gravity. Thebeads were further washed by the slow addition of 8 liters of deionizedwater. The beads were further separated from the wash water by applyingaspirator vacuum to the funnel. The beads were dried at 140° C. in anoven and were placed in a sealed jar to cool prior to use.

A titania dispersion was prepared by mixing with a high shear mixer (IKAUltra Turrax T18 mixer; IKA Works, Inc., Wilmington, Del.) 10.0 g ofHombikat UV100 titania (Sachtleben Chemie Gmbh, Duisburg, Germany) with10.1 g of the γ-Fe₂O₃ colorant dispersion, 5.0 g potassium carbonate and90.2 g of deionized water. This mixture was applied to 215.0 g of thewashed Alcoa 450 alumina beads by spraying the dispersion through aQorpak finger-actuated trigger-sprayer (Qorpak, Bridgeville, Pa.) as afine mist onto a shallow bed of the Alcoa 450 alumina beads uniformlyspread out on a glass tray. The bed of alumina beads was mixed gentlyusing a rubber policeman after every 2 sprays to provide a uniformcoating of the beads with the titania dispersion. After application, thecoated beads were dried at 100° C. in an oven for one hour and then at140° C. for 2 hours.

A catalyst sample of this material was prepared by sputter coating a 300ml sample of the titania-coated Alcoa 450 alumina beads with goldaccording to deposition conditions: cathodic power 0.03 kW; sputtertime—60 minutes; blade gap 6.9 mm; gold target weight loss 1.14 g.

100 mL of the sample was tested (test procedure 1) under the sameconditions as example 3. ΔP of the sample in the fixture was 5.2 mm H₂Oat 85 L/min. Test RH was >90%. The test results are shown in FIG. 13.The data depicted by the open diamonds show the temperature data at theoutlet, and the data depicted by the black triangles show the detectedCO data at the outlet. CO concentration at the outlet of the filter(solid triangles) increased slowly from 0 to about 12 ppm over an hour.The temperature of the air at the filter outlet (open circles) spiked toabout 54° C. when the wet challenge air contacted the dry catalyst. Thisspike in temperature did not occur with the activated catalyst sample ofexample 3 in which the host was carbon. Temperature then slowly droppedto about 36.5° C. over an hour.

At about 60 minutes into the test the inlet CO concentration wasincreased suddenly to 10,000 ppm. CO concentration at the outlet spikedupward significantly before dropping back to about 6 ppm. The recoverytime for this sample was slower than that for Example 3.

The outlet CO concentration during this test was also monitored with aB&K gas analyzer. This device can analyze the CO content of a sampleabout every minute while the GC takes about 3 minutes to complete ananalysis. The maximum CO concentration measured by the B&K was 1170 ppm,which is an order of magnitude greater than the more moderate spikeobserved in Example 3. Air temperature at the outlet began to risesharply and reached about 105° C.

After about 90 minutes, the CO inlet concentration was dropped back to1200 ppm. The catalyst continued to function even as the temperaturedropped to less than 50° C. The catalyst increased in weight by 7.7 gduring the test. The alumina catalyst in this example allowed more CO toslip through the bed before low CO outlet concentrations werere-established than did the activated carbon catalyst in example 3.

Example 9 Sputtered Au on TiO₂/Kuraray GG Carbon: Effect on PhysicalAdsorption of Cyclohexane

The catalyst was tested against a cyclohexane challenge to evaluate itscapability for physical adsorption of organic vapors. Untreated KurarayGG 12×20 activated carbon was tested as a control. A titania dispersionwas prepared by mixing with a high shear mixer (IKA Ultra Turrax T18mixer; IKA Works, Inc., Wilmington, Del.) 10.0 g of Hombikat UV100titania (Sachtleben Chemie Gmbh, Duisburg, Germany) with 60. g ofdeionized water. This mixture was applied to 100 grams of 12×20 meshKuraray GG carbon particles by spraying the titania dispersion through aQorpak finger-actuated trigger-sprayer (Qorpak, Bridgeville, Pa.) as afine mist onto a shallow bed of the GG carbon uniformly spread out on aglass tray. The bed of carbon particles was mixed gently using a rubberspatula after each 1-2 sprays to provide a uniform coating of thetitania dispersion on the carbon particles. After the addition of thetitania, the titania-coated particles were dried at 100° C. in an ovenfor 30 minutes and then at 150° C. for 1 hour.

5 mL of each (2.1 g) were tested against a 1000 ppm cyclohexanechallenge at 1.6 L/min using the method described in test procedure 2.The GC was equipped with a 6 ft 10% SE-30 on Chromosorb W—HP 80/100column for analysis of cyclohexane instead of CO.

The results are shown in FIG. 14. The abilities of both samples toprotect against cyclohexane are quite similar. Surprisingly, the titaniacoating of the composite catalyst system did not unduly interfere withthe fast transport of organic vapor molecules into the internal porevolume of the activated carbon support host. This illustrates that thecarbon host incorporated into composite catalysts of the presentinvention retain appreciable capacity for physical adsorption. This dualfunctionality is advantageous for use in a respirator filter.

Other embodiments of this invention will be apparent to those skilled inthe art upon consideration of this specification or from practice of theinvention disclosed herein. Various omissions, modifications, andchanges to the principles and embodiments described herein may be madeby one skilled in the art without departing from the true scope andspirit of the invention which is indicated by the following claims.

1. A method of making a catalyst system comprising nanoporous compositeparticles, the method comprising the steps of: forming catalyticallyactive elemental gold clusters of dimensions of about 0.5 nm to about 50nm on a plurality of guest particles by a process consisting essentiallyof physical vapor deposition; and, incorporating the plurality of guestparticles onto carbonaceous host particles to form a plurality ofnanoporous composite particles, wherein the ratio of the volume averageparticle size of the host particles to the guest particles is greaterthan about 3:1.
 2. The method of claim 1, wherein the guest particlescomprise nanoporosity.
 3. The method of claim 1, wherein aggregates ofthe guest particles comprise nanoporosity.
 4. The method of claim 1,wherein the carbonaceous host particles comprise nanoporosity.
 5. Themethod of claim 1, wherein the carbonaceous host particles compriseactivated carbon particles.
 6. The method of claim 1, wherein the guestparticles comprise titania particles.
 7. The method of claim 1, furthercomprising the step of impregnating a water soluble metal salt onto atleast the guest particles, wherein said impregnation occurs prior to thephysical vapor deposition of the gold.
 8. The method of claim 1, furthercomprising the step of impregnating a water soluble metal salt onto atleast the carbonaceous host particles, wherein said impregnation occursafter the physical vapor deposition of the gold.
 9. The method of claim8, wherein the water soluble salt is selected from the group consistingof alkali metal salts, alkaline earth salts, and combinations thereof.10. The method of claim 1, wherein the gold is physically vapordeposited on the guest particles before the guest particles areincorporated onto the carbonaceous host particles to form the pluralityof nanoporous composite particles.
 11. The method of claim 1, whereinthe gold is physically vapor deposited on the guest particles after theguest particles are incorporated onto the carbonaceous host particles toform the plurality of nanoporous composite particles.
 12. The method ofclaim 1, wherein the forming of the catalytically active elemental goldclusters of dimensions of about 0.5 nm to about 50 nm on the pluralityof guest particles is by a process consisting essentially of physicalvapor deposition of elemental gold.
 13. The method of claim 1, whereinthe guest particles and the carbonaceous host particles are mixed andcomminuted during at least a portion of the time in which physical vapordeposition of the gold occurs.
 14. The method of claim 1, wherein thenanoporous composite particles are prepared by a method comprisingphysically mixing the guest particles and the host particles.
 15. Amethod of oxidizing CO, comprising the steps of: providing a catalystsystem comprising nanoporous composite particles, wherein the nanoporouscomposite particles each comprise a plurality of guest particles on acarbonaceous host particle wherein the ratio of the volume averageparticle size of the carbonaceous host particles to the guest particlesis greater than about 3:1 and wherein the guest particles comprisecatalytically active elemental gold clusters of dimensions of about 0.5nm to about 50 nm formed on the guest particles by a process consistingessentially of physical vapor deposition; and, causing the catalystsystem to catalytically contact the CO.
 16. The method of claim 15wherein the oxidizing of CO occurs in a respiratory protection system.17. The method of claim 16 wherein the oxidizing of CO occurs in apersonal respiratory protection system.
 18. The method of claim 17,wherein the personal respiratory protection system is selected from thegroup consisting of a mask and an escape hood.
 19. The method of claim15, wherein the carbonaceous host particles comprise activated carbonparticles.
 20. The method of claim 15, wherein the guest particlescomprise titania particles.
 21. The method of claim 20 wherein thecarbonaceous host particles comprise activated carbon particles.
 22. Themethod of claim 15, wherein the nanoporous composite particles compriseat least one water soluble metal salt.
 23. The method of claim 22,wherein the water soluble metal salt is selected from the groupconsisting of an alkali metal salt, an alkaline earth metal salt, andmixtures thereof.
 24. The method of claim 23 wherein the guest particlescomprise titania particles and the carbonaceous host particles compriseactivated carbon particles.